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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-K

ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF

THE SECURITIES EXCHANGE ACT OF 1934

For the fiscal year ended December 31, 2004

Commission File Number 1-7850

SOUTHWEST GAS CORPORATION

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (702) 876-7237

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes x No ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K. ¨

Indicate by check mark whether the registrant is an accelerated filer. Yes x No ¨

Aggregate market value of the voting and non-voting common stock held by nonaffiliates of the registrant:

$848,203,765 as of June 30, 2004

The number of shares outstanding of common stock:

Common Stock, $1 Par Value, 37,208,075 shares as of March 1, 2005

DOCUMENTS INCORPORATED BY REFERENCE

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TABLE OF CONTENTS

PART I

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PART I

Item 1. BUSINESS

Southwest Gas Corporation (the “Company”) was incorporated, effective March 1931, under the laws of the state of California. The Company is comprised of two business segments: natural gas operations (“Southwest” or the “natural gas operations” segment) and construction services. Southwest is engaged in the business of purchasing, transporting, and distributing natural gas in portions of Arizona, Nevada, and California. Southwest is the largest distributor in Arizona, selling and transporting natural gas in most of central and southern Arizona, including the Phoenix and Tucson metropolitan areas. Southwest is also the largest distributor and transporter of natural gas in Nevada, serving the Las Vegas metropolitan area and northern Nevada. In addition, Southwest distributes and transports natural gas in portions of California, including the Lake Tahoe area and the high desert and mountain areas in San Bernardino County.

Northern Pipeline Construction Co. (“NPL” or the “construction services” segment), a wholly owned subsidiary, is a full-service underground piping contractor that provides utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems.

Financial information concerning the Company’s business segments is included in Note 11 of the Notes to Consolidated Financial Statements which is included in the 2004 Annual Report to Shareholders and is incorporated herein by reference.

The Company maintains a website (www.swgas.com) for the benefit of shareholders, investors, customers, and other interested parties. The Company makes its annual report on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K, and all amendments to those reports available, free of charge, through its website as soon as reasonably practicable after such material is electronically filed with, or furnished to, the Securities and Exchange Commission (“SEC”). The Company’s Corporate Governance Guidelines, Code of Business Conduct and Ethics, and charters of the nominating and corporate governance, audit, and compensation committees of the board of directors are also available on the website and are available in print by request.

NATURAL GAS OPERATIONS

General Description

Southwest is subject to regulation by the Arizona Corporation Commission (“ACC”), the Public Utilities Commission of Nevada (“PUCN”), and the California Public Utilities Commission (“CPUC”). These commissions regulate public utility rates, practices, facilities, and service territories in their respective states. The CPUC also regulates the issuance of all securities by the Company, with the exception of short-term borrowings. Certain accounting practices, transmission facilities, and rates are subject to regulation by the Federal Energy Regulatory Commission (“FERC”). NPL is not regulated by the state utilities commissions in any of its operating areas.

As of December 31, 2004, Southwest purchased, transported, and distributed natural gas to 1,613,000 residential, commercial, and industrial customers in geographically diverse portions of Arizona, Nevada, and California. There were 82,000 customers added to the system during 2004.

The table below lists the percentage of operating margin (operating revenues less net cost of gas) by major customer class for the years indicated:

Southwest is not dependent on any one or a few customers to the extent that the loss of any one or several would have a significant adverse impact on earnings.

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Transportation of customer-secured gas to end-users accounted for 50 percent of total system throughput in 2004. Although the volumes were significant, these customers provide a much smaller proportionate share of operating margin. Customers who utilized this service transported 126 million dekatherms in 2004, 134 million dekatherms in 2003, and 133 million dekatherms in 2002.

The demand for natural gas is seasonal. Variability in weather from normal temperatures can materially impact results of operations. It is the opinion of management that comparisons of earnings for interim periods do not reliably reflect overall trends and changes in operations. Also, earnings for interim periods can be significantly affected by the timing of general rate relief.

Rates and Regulation

Rates that Southwest is authorized to charge its distribution system customers are determined by the ACC, PUCN, and CPUC in general rate cases and are derived using rate base, cost of service, and cost of capital experienced in an historical test year, as adjusted in Arizona and Nevada, and projected for a future test year in California. The FERC regulates the northern Nevada transmission and liquefied natural gas (“LNG”) storage facilities of Paiute Pipeline Company (“Paiute”), a wholly owned subsidiary, and the rates it charges for transportation of gas directly to certain end-users and to various local distribution companies (“LDCs”). The LDCs transporting on the Paiute system are: Sierra Pacific Power Company (serving Reno and Sparks, Nevada), Avista Utilities (serving South Lake Tahoe, California), and Southwest Gas Corporation (serving Truckee and North Lake Tahoe, California and various locations throughout northern Nevada). In July 2004, Southwest announced an agreement to purchase the Avista natural gas distribution properties in South Lake Tahoe. The purchase, subject to regulatory approval, is expected to close in the second quarter of 2005.

Rates charged to customers vary according to customer class and rate jurisdiction and are set at levels that are intended to allow for the recovery of all prudently incurred costs, including a return on rate base sufficient to pay interest on debt, preferred securities distributions, and a reasonable return on common equity. Rate base consists generally of the original cost of utility plant in service, plus certain other assets such as working capital and inventories, less accumulated depreciation on utility plant in service, net deferred income tax liabilities, and certain other deductions. Rate schedules in California and Arizona service areas currently contain purchased gas adjustment clauses, which allow Southwest to file for rate adjustments as the cost of purchased gas changes. In Arizona, Southwest adjusts rates monthly for changes in purchased gas costs, within pre-established limits. In California, a monthly gas cost adjustment based on forecasted monthly prices is used to adjust rates. In Nevada, the PUCN issued an order in October 2004 instructing Southwest to eliminate the purchased gas adjustment provisions in its tariff and instead account for gas costs as provided under the deferred energy provisions of the Nevada Administrative Code. These provisions result in little difference in the method used to account for or report purchased gas costs, including the ability of the Company to defer over or under-collections of gas costs to balancing accounts. The changes become effective at the time Southwest makes its next purchased gas cost adjustment filing. The Nevada Administrative Code will continue to require at least an annual filing to adjust for changes in purchased gas costs. Deferred energy and purchased gas adjustment (collectively “PGA”) rate changes affect cash flows but have no direct impact on profit margin. Filings to change rates in accordance with PGA clauses are subject to audit by the appropriate state regulatory commission staff. Information with respect to recent general rate cases and PGA and deferred energy filings is included in the Rates and Regulatory Proceedings section of Management’s Discussion and Analysis (“MD&A”) in the 2004 Annual Report to Shareholders.

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The table below lists the docketed general rate filings last initiated and the status of such filing within each ratemaking area:

Demand for Natural Gas

Deliveries of natural gas by Southwest are made under a priority system established by state regulatory commissions. The priority system is intended to ensure that the gas requirements of higher-priority customers, primarily residential customers and other customers who use 500 therms of gas per day or less, are fully satisfied on a daily basis before lower-priority customers, primarily electric utility and large industrial customers able to use alternative fuels, are provided any quantity of gas or capacity.

Demand for natural gas is greatly affected by temperature. On cold days, use of gas by residential and commercial customers may be as much as six times greater than on warm days because of increased use of gas for space heating. To fully satisfy this increased high-priority demand, gas is withdrawn from storage in certain service areas, or peaking supplies are purchased from suppliers. If necessary, service to interruptible lower-priority customers may be curtailed to provide the needed delivery system capacity. No curtailment occurred during the latest peak heating season. Southwest maintains no significant backlog on its orders for gas service.

Natural Gas Supply

Southwest is responsible for acquiring (purchasing) and arranging delivery of (transporting via interstate pipelines) natural gas to its system for all sales customers.

The primary objective of Southwest with respect to acquiring gas supply is to ensure that adequate, as well as best cost economical, supplies of natural gas are available from reliable sources. Gas is acquired from a wide variety of sources and a mix of purchase provisions, including spot market purchases and firm supplies with a variety of terms. During 2004, Southwest acquired gas supplies from 39 suppliers. Southwest constantly monitors the number of suppliers, their quality and their contribution to the overall customer portfolio. New suppliers are contracted whenever possible and solicitations for supplies are extended to the largest possible list of suppliers. Competitive pricing, flexibility in meeting Southwest requirements, and aggressive participation by suppliers who have demonstrated reliability of service are key to their successful inclusion in the annual portfolio mix. The goal of this practice is to mitigate the risk of nonperformance by any one supplier and insure competitive prices for customer supplies.

Balancing reliable supply assurances with the associated costs results in a continually changing mix of purchase provisions within the supply portfolios. To address the unique requirements of its various market areas, Southwest assembles and administers a separate natural gas supply portfolio for each of its jurisdictional areas. Firm and spot market natural gas purchases are made in a competitive bid environment. Southwest has experienced price volatility over the past five years, as the weighted average delivered cost of natural gas has ranged from a low of 38 cents per therm in 2002 to a high of 57 cents per therm in 2004. To mitigate customer exposure to market price volatility, Southwest continues to purchase a significant percentage of its forecasted annual normal weather requirements under firm, fixed-price arrangements that are secured periodically throughout the year.

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The firm, fixed-price arrangements are structured such that a stated volume of gas is required to be scheduled by Southwest and delivered by the supplier. If the gas is not needed by Southwest or cannot be procured by the supplier, the contract provides for fixed or market-based penalties to be paid by the non-performing party.

In managing its gas supply portfolios, Southwest uses the fixed-price arrangements noted above, but does not currently utilize other stand-alone derivative financial instruments. Southwest is currently considering using stand-alone derivatives to hedge against possible price volatility. However, any such change would be communicated to Southwest’s various regulatory commissions, and costs of such derivative financial instruments would be pursued as part of the PGA mechanisms for recovery from customers in each jurisdiction.

Storage capability can influence the average annual price of gas, as storage allows a company to purchase natural gas in larger quantities during the off-peak season and store it for use in high demand periods when prices may be greater or supplies/capacity tighter. Southwest currently has no storage availability in its Arizona or southern Nevada rate jurisdictions. Limited storage capabilities exist in southern and northern California and northern Nevada. A contract with Southern California Gas Company is intended for delivery only within Southwest’s southern California rate jurisdiction. In addition, a contract with Paiute for its LNG facility in northern Nevada and northern California allows for peaking capability only. Gas is purchased for injection during the off-peak period for use in the high demand months, but is limited in its impact on the overall price. Paiute purchased the LNG facility, which it previously leased, in December 2004.

Gas supplies for the southern system of Southwest (Arizona, southern Nevada, and southern California properties) are primarily obtained from producing regions in Colorado and New Mexico (San Juan basin), Texas (Permian basin), and Rocky Mountain areas. For its northern system (northern Nevada and northern California properties), Southwest primarily obtains gas from Rocky Mountain producing areas and from Canada.

Southwest arranges for transportation of gas to its Arizona, Nevada, and California service territories through the pipeline systems of El Paso Natural Gas Company (“El Paso”), Kern River Gas Transmission Company (“Kern River”), Transwestern Pipeline Company, Northwest Pipeline Corporation, Tuscarora Gas Pipeline Company, Southern California Gas Company and Paiute. Supply and pipeline capacity availability on both short- and long-term bases is continually monitored by Southwest to ensure the reliability of service to its customers. Southwest currently receives firm transportation service, both on a short- and long-term basis, for all of its service territories on the pipeline systems noted above, and also has interruptible contracts in place that allow additional capacity to be acquired should an unforeseen need arise.

The Company believes that the current level of contracted firm interstate capacity is sufficient to serve each of its service territories. As the need arises to acquire additional capacity on one of the interstate pipeline transmission systems, primarily due to customer growth, Southwest will continue to consider available options to obtain that capacity, either through the use of firm contracts with a pipeline company or by purchasing capacity on the open market.

Southwest is dependent upon the El Paso pipeline system for the transportation of gas to virtually all of its Arizona service territories. Historically, Southwest received transportation service from El Paso to its Arizona service territories under a full requirements contract. Under full requirements service, El Paso was obligated to transport all of a customer’s gas requirements each day, and the customer was obligated to have El Paso, and only El Paso, transport its requirements. Virtually all of El Paso’s customers in Arizona, New Mexico, and Texas have been full requirements customers, while El Paso has transported gas for its customers in California and Nevada subject to a specific maximum daily quantity, or contract demand limitation.

Over the past several years, the FERC has examined capacity allocation issues on the El Paso system in several proceedings. This examination resulted in a series of orders by the FERC in which all of the major full requirements transportation service agreements on the El Paso system, including the agreement by which Southwest obtained the transportation of gas supplies to its Arizona service areas, were converted to contract demand-type service agreements, with fixed maximum service limits, effective September 2003. At that time, all of the transportation capacity on the system was allocated among the shippers. In order to help ensure that the converting full requirements shippers would have adequate capacity to meet their needs, El Paso was authorized to expand the capacity on its system by adding compression.

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Since 2003, the FERC has reviewed issues related to the implementation of the full requirements conversion. Parties, including Southwest, filed petitions for judicial review of the FERC orders mandating the conversion. In December 2004, the United States Court of Appeals denied a petition seeking to reverse the prior FERC order that converted the agreements to contract demand. As a result, Southwest plans to pursue a reallocation of shipper costs at the United States Court of Appeals level based upon the contract demand quantities. However, no additional actions are anticipated on the capacity allocation issue.

Management believes adequate capacity exists for the upcoming 2005/2006 heating season. Additional costs may be incurred to acquire capacity in the future as a result of the FERC order. However, it is anticipated that any additional costs will be collected from customers through the PGA mechanism.

Competition

Electric utilities are the principal competitors of Southwest for the residential and small commercial markets throughout its service areas. Competition for space heating, general household, and small commercial energy needs generally occurs at the initial installation phase when the customer/builder typically makes the decision as to which type of equipment to install and operate. The customer will generally continue to use the chosen energy source for the life of the equipment. As a result of its success in these markets, Southwest has experienced consistent growth among the residential and small commercial customer classes.

Unlike residential and small commercial customers, certain large commercial, industrial, and electric generation customers have the capability to switch to alternative energy sources. To date, Southwest has been successful in retaining most of these customers by setting rates at levels competitive with alternative energy sources such as electricity, fuel oils, and coal. However, increases in natural gas prices, if sustained for an extended period of time, may impact Southwest’s ability to retain some of these customers. Overall, management does not anticipate any material adverse impact on operating margin from fuel switching.

Southwest continues to compete with interstate transmission pipeline companies, such as El Paso, Kern River, and Tuscarora Gas Pipeline Company, to provide service to certain large end-users. End-use customers located in proximity to these interstate pipelines pose a potential bypass threat. Southwest attempts to closely monitor each customer situation and provide competitive service in order to retain the customer. Southwest has remained competitive through the use of negotiated transportation contract rates, special long-term contracts with electric generation and cogeneration customers, and other tariff programs. These competitive response initiatives have mitigated the loss of margin earned from large customers.

Environmental Matters

Federal, state, and local laws and regulations governing the discharge of materials into the environment have had little direct impact upon Southwest. Environmental efforts, with respect to matters such as protection of endangered species and archeological finds, have increased the complexity and time required to obtain pipeline rights-of-way and construction permits. However, increased environmental legislation and regulation are also beneficial to the natural gas industry. Because natural gas is one of the most environmentally safe fossil fuels currently available, its use can help energy users to comply with stricter environmental standards.

Employees

At December 31, 2004, the natural gas operations segment had 2,548 regular full-time equivalent employees, of which 481 full-time equivalent non-exempt employees in central Arizona were represented by the International Brotherhood of Electrical Workers. No other natural gas operations segment employees are represented by a union. Southwest believes it has a good relationship with its employees and that compensation, benefits, and working conditions afforded its employees are comparable to those generally found in the utility industry.

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CONSTRUCTION SERVICES

NPL is a full-service underground piping contractor that provides utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems. NPL contracts primarily with LDCs to install, repair, and maintain energy distribution systems from the town border station to the end-user. The primary focus of business operations is main and service replacement as well as new business installations. Construction work varies from relatively small projects to the piping of entire communities. Construction activity is seasonal in most areas. Peak construction periods are the summer and fall months in colder climate areas, such as the midwest. In the warmer climate areas, such as the southwestern United States, construction continues year round.

NPL business activities are often concentrated in utility service territories where existing energy lines are scheduled for replacement. An LDC will typically contract with NPL to provide pipe replacement services and new line installations. Contract terms generally specify unit-price or fixed-price arrangements. Unit-price contracts establish prices for all of the various services to be performed during the contract period. These contracts often have annual pricing reviews. During 2004, approximately 92 percent of revenue was earned under unit-price contracts. As of December 31, 2004 no significant backlog existed with respect to outstanding construction contracts.

Materials used by NPL in its pipeline construction activities are typically specified, purchased, and supplied by NPL’s customers. Construction contracts also contain provisions which make customers generally liable for remediating environmental hazards encountered during the construction process. Such hazards might include digging in an area that was contaminated prior to construction, finding endangered animals, digging in historically significant sites, etc. Otherwise, NPL’s operations have minimal environmental impact (dust control, normal waste disposal, handling harmful materials, etc.).

Competition within the industry has traditionally been limited to several regional competitors in what has been a largely fragmented industry. Several national competitors also exist within the industry. NPL currently operates in approximately 16 major markets nationwide. Its customers are the primary LDCs in those markets. During 2004, NPL served 34 major customers, with Southwest accounting for approximately 29 percent of NPL revenues. With the exception of two other customers that in total accounted for approximately 24 percent of revenue, no other customer had a relatively significant contribution to NPL revenues.

Employment fluctuates between seasonal construction periods, which are normally heaviest in the summer and fall months. At December 31, 2004, NPL had 1,953 regular full-time equivalent employees. Employment peaked in November 2004 when there were 2,168 employees. Most employees are represented by unions and are covered by collective bargaining agreements, which is typical of the utility construction industry.

Operations are conducted from 16 field locations with corporate headquarters located in Phoenix, Arizona. Buildings are normally leased from third parties. The lease terms are typically five years or less. Field location facilities consist of a small building for repairs and land to store equipment.

NPL is not directly affected by regulations promulgated by the ACC, PUCN, CPUC or FERC in its construction services. NPL is an unregulated construction subsidiary of Southwest Gas Corporation. However, because NPL performs work for the regulated natural gas segment of the Company, its construction costs are subject indirectly to “prudency reviews” just as any other capital work that is performed by third parties or directly by Southwest. However, such “prudency reviews” would not bring NPL under the regulatory jurisdiction of any of the commissions noted above.

COMPANY RISK FACTORS

Although the Company is not able to predict all factors that may affect future results, described below are some of the risk factors identified by the Company that may have a negative impact on our future financial performance or affect whether we achieve the goals or expectations expressed or implied in any forward-looking statements contained herein. Unless indicated otherwise, references below to “we,” “us” and “our” should be read to refer to Southwest Gas Corporation and its subsidiaries.

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Our liquidity, and in certain circumstances our earnings, may be reduced during periods in which natural gas prices are rising significantly or are more volatile.

Rate schedules in each of our service territories contain purchased gas adjustment clauses which permit us to file for rate adjustments to recover increases in the cost of purchased gas. Increases in the cost of purchased gas have no direct impact on our profit margins, but do affect cash flows and can therefore impact the amount of our capital resources. We have used short-term borrowings in the past to temporarily finance increases in purchased gas costs, and we expect to do so during 2005, if the need again arises.

We may file requests for rate increases to cover the rise in the costs of purchased gas. Due to the nature of the regulatory process, there is a risk of a disallowance of full recovery of these costs during any period in which there has been a substantial run-up of these costs or our costs are more volatile. Any disallowance of purchased gas costs may reduce cash flow and earnings.

Increases in the cost of natural gas may arise from a variety of factors, including weather, changes in demand, the level of production and availability of natural gas, transportation constraints, transportation capacity cost increases, federal and state energy and environmental regulation and legislation, the degree of market liquidity, natural disasters, wars and other catastrophic events, and the success of our strategies in managing price risk such as the possible future use of stand-alone derivative instruments to hedge against potential price increases.

Governmental policies and regulatory actions can reduce our earnings.

Governmental policies and regulatory actions, including those of the ACC, the CPUC, the FERC, and the PUCN relating to allowed rates of return, rate structure, purchased gas and investment recovery, operation and construction of facilities, present or prospective wholesale and retail competition, changes in tax laws and policies, and changes in and compliance with environmental and safety laws and policies, can reduce our earnings. Risks and uncertainties relating to delays in obtaining regulatory approvals, conditions imposed in regulatory approvals, or determinations in regulatory investigations can also impact financial performance.

We are unable to predict what types of conditions might be imposed on Southwest or what types of determinations might be made in pending or future regulatory proceedings or investigations. We nevertheless believe that it is not uncommon for conditions to be imposed in regulatory proceedings, for Southwest to agree to conditions as part of a settlement of a regulatory proceeding, or for determinations to be made in regulatory investigations that will reduce our earnings and liquidity. For example, we may request recovery of a particular operating expense in a general rate case filing that a regulator disallows.

Significant customer growth in Arizona and Nevada could strain our capital resources.

We continue to experience significant population and customer growth throughout our service territories. During 2004, we added 82,000 customers, a five percent growth rate. Over the past ten years, customer growth has averaged five percent per year. This growth has required large amounts of capital to finance the investment in new transmission and distribution plant. In 2004, our natural gas construction expenditures totaled $253 million (excluding a $22 million LNG facility purchase). Approximately 75 percent of these current-period expenditures represented new construction, and the balance represented costs associated with routine replacement of existing transmission, distribution, and general plant.

Cash flows from operating activities (net of dividends) have been inadequate, and are expected to continue to be inadequate, to fund all necessary capital expenditures. We have funded this shortfall through the issuance of additional debt and equity securities, and expect to continue to do so. However, our ability to issue additional securities is dependent upon, among other things, conditions in the capital markets, regulatory authorizations, and our level of earnings.

Significant customer growth in Arizona and Nevada could also impact earnings.

Our ability to earn the rates of return authorized by the ACC and the PUCN is also more difficult because of significant customer growth. The rates we charge our distribution customers in Arizona and Nevada are derived using rate base, cost of service, and cost of capital experienced in an historical test year, as adjusted. This results in “regulatory

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lag” which delays our recovery of some of the costs of capital improvements and operating costs from customers in Arizona and Nevada.

Our earnings are greatly affected by variations in temperature during the winter heating season.

The demand for natural gas is seasonal and is greatly affected by temperature. Variability in weather from normal temperatures can materially impact results of operations. On cold days, use of gas by residential and commercial customers may be as much as six times greater than on warm days because of the increased use of gas for space heating. Weather has been and will continue to be one of the dominant factors in our financial performance.

Uncertain economic conditions may affect our ability to finance capital expenditures.

Our ability to finance capital expenditures and other matters will depend upon general economic conditions in the capital markets. The direction of interest rates is uncertain. Declining interest rates are generally believed to be favorable to utilities while rising interest rates are believed to be unfavorable because of the high capital costs of utilities. In addition, our authorized rate of return is based upon certain assumptions regarding interest rates. If interest rates are lower than assumed rates, our authorized rate of return in the future could be reduced. If interest rates are higher than assumed rates, it will be more difficult for us to earn our currently authorized rate of return.

We are responsible for insurance claims above $1 million per incident plus payment of the first $10 million in aggregate claims above $1 million due to a change in our insurance coverage.

We maintain liability insurance for various risks associated with the operation of our natural gas pipelines and facilities. In connection with these liability insurance policies, we have been responsible for an initial deductible or self-insured retention amount per incident, after which the insurance carriers would be responsible for amounts up to the policy limits. For the policy year August 2004 to July 2005, the self-insured retention amount associated with general liability claims increased from $1 million per incident to $1 million per incident plus payment of the first $10 million in aggregate claims above $1 million in the policy year. This increase primarily occurred due to several significant claims during the last ten years. We cannot predict the likelihood that any future claim will exceed $1 million; however, a large claim would reduce our earnings.

A significant reduction in our credit ratings could materially and adversely affect our business, financial condition and results of operations.

We cannot be certain that any of our current ratings will remain in effect for any given period of time or that a rating will not be lowered or withdrawn entirely by a rating agency if, in its judgment, circumstances in the future so warrant. Any downgrade could increase our borrowing costs, which would diminish our financial results. We would likely be required to pay a higher interest rate in future financings, and our potential pool of investors and funding sources could decrease. A downgrade could require additional support in the form of letters of credit or cash or other collateral and otherwise adversely affect our business, financial condition and results of operations.

Item 2. PROPERTIES

The plant investment of Southwest consists primarily of transmission and distribution mains, compressor stations, peak shaving/storage plants, service lines, meters, and regulators, which comprise the pipeline systems and facilities located in and around the communities served. Southwest also includes other properties such as land, buildings, furnishings, work equipment, vehicles, and software systems in plant investment. The northern Nevada and northern California properties of Southwest are referred to as the northern system; the Arizona, southern Nevada, and southern California properties are referred to as the southern system. Several properties are leased by Southwest, including a portion of the corporate headquarters office complex located in Las Vegas, Nevada and the administrative offices in Phoenix, Arizona. Total gas plant, exclusive of leased property, at December 31, 2004 was $3.3 billion, including construction work in progress. It is the opinion of management that the properties of Southwest are suitable and adequate for its purposes.

Substantially all gas main and service lines are constructed across property owned by others under right-of-way grants obtained from the record owners thereof, on the streets and grounds of municipalities under authority conferred by

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franchises or otherwise, or on public highways or public lands under authority of various federal and state statutes. None of the numerous county and municipal franchises are exclusive, and some are of limited duration. These franchises are renewed regularly as they expire, and Southwest anticipates no serious difficulties in obtaining future renewals.

With respect to the right-of-way grants, Southwest has had continuous and uninterrupted possession and use of all such rights-of-way, and the associated gas mains and service lines, commencing with the initial stages of the construction of such facilities. Permits have been obtained from public authorities and other governmental entities in certain instances to cross or to lay facilities along roads and highways. These permits typically are revocable at the election of the grantor and Southwest occasionally must relocate its facilities when requested to do so by the grantor. Permits have also been obtained from railroad companies to cross over or under railroad lands or rights-of-way, which in some instances require annual or other periodic payments and are revocable at the election of the grantors.

Southwest operates two primary pipeline transmission systems: (i) a system owned by Paiute extending from the Idaho-Nevada border to the Reno, Sparks, and Carson City areas and communities in the Lake Tahoe area in both California and Nevada and other communities in northern and western Nevada; and (ii) a system extending from the Colorado River at the southern tip of Nevada to the Las Vegas distribution area.

The Company previously leased a LNG facility and approximately 61 miles of transmission main on its northern Nevada system. These storage and transmission facilities provide peaking capabilities during high demand months. Paiute purchased the LNG facility and the associated transmission main in December 2004.

The following map shows the locations of major Southwest facilities and transmission lines, and principal communities to which Southwest supplies gas either as a wholesaler or distributor. The map also shows major supplier transmission lines that are interconnected with the Southwest systems.

Information on properties of NPL can be found on page 6 of this Form 10-K under Construction Services.

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Item 3. LEGAL PROCEEDINGS

The Company is named as a defendant in various legal proceedings. The ultimate dispositions of these proceedings are not presently determinable; however, it is the opinion of management that none of this litigation individually or in the aggregate will have a material adverse impact on the Company’s financial position or results of operations.

Item 4. SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS

None.

PART II

The principal market on which the common stock of the Company is traded is the New York Stock Exchange. At March 1, 2005, there were 24,174 holders of record of common stock, and the market price of the common stock was $25.29. The quarterly market price of, and dividends on, Company common stock required by this item are included in the 2004 Annual Report to Shareholders filed as an exhibit hereto and incorporated herein by reference.

The Company has a common stock dividend policy which states that common stock dividends will be paid at a prudent level that is within the normal dividend payout range for its respective businesses, and that the dividend will be established at a level considered sustainable in order to minimize business risk and maintain a strong capital structure throughout all economic cycles. The quarterly common stock dividend was 20.5 cents per share throughout 2003 and 2004. The dividend of 20.5 cents per share has been paid quarterly since September 1994.

Item 6. SELECTED FINANCIAL DATA

Information required by this item is included in the 2004 Annual Report to Shareholders and is incorporated herein by reference.

Information required by this item is included in the 2004 Annual Report to Shareholders and is incorporated herein by reference.

Item 7A. QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK

Information required by this item is included in the 2004 Annual Report to Shareholders under the heading “Management’s Discussion and Analysis” and under Notes 6 and 7 of the Notes to Consolidated Financial Statements and is incorporated herein by reference. Other risk information is included under the heading “Company Risk Factors” in Item 1. Business of this report.

Item 8. FINANCIAL STATEMENTS AND SUPPLEMENTARY DATA

The Consolidated Financial Statements of Southwest Gas Corporation and Notes thereto, together with the report of PricewaterhouseCoopers LLP, are included in the 2004 Annual Report to Shareholders and are incorporated herein by reference.

None.

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Item 9A. CONTROLS AND PROCEDURES

Disclosure Controls and Procedures

The Company has established disclosure controls and procedures that are designed to provide reasonable assurance that information required to be disclosed in reports filed or submitted under the Securities Exchange Act of 1934 is recorded, processed, summarized, and reported within the time periods specified in the SEC’s rules and forms. A control system, no matter how well conceived and operated, can provide only reasonable, not absolute, assurance that the objectives of the control system are met. Further, the design of a control system must reflect the fact that there are resource constraints, and benefits of controls must be considered relative to their costs. Additionally, controls can be circumvented by the individual acts of some persons, by collusion of two or more people, or management override of the control. Because of the inherent limitations in a cost-effective control system, misstatements due to error or fraud may occur and not be detected.

Based on the most recent evaluation, as of December 31, 2004, management of the Company, including the Chief Executive Officer and Chief Financial Officer, believe the Company’s disclosure controls and procedures are effective at attaining the level of reasonable assurance noted above.

Internal Control Over Financial Reporting

The report of management of the Company required to be reported herein is incorporated by reference to the information reported in the 2004 Annual Report to Shareholders under the caption “Management’s Report on Internal Control Over Financial Reporting” on page 69.

The Attestation Report of the Registered Public Accounting Firm required to be reported herein is incorporated by reference to the information reported in the 2004 Annual Report to Shareholders under the caption “Report of Independent Registered Public Accounting Firm” on page 70.

There has been no change in our internal control over financial reporting that occurred during our most recent fiscal quarter that has materially affected or is reasonably likely to materially affect our internal control over financial reporting.

Item 9B. OTHER INFORMATION

None.

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PART III

Item 10. DIRECTORS AND EXECUTIVE OFFICERS OF THE REGISTRANT

(a) Identification of Directors. Information with respect to Directors is set forth under the heading “Election of Directors” in the definitive 2005 Proxy Statement, which by this reference is incorporated herein.

(b) Identification of Executive Officers. The name, age, position, and period position held during the last five years for each of the Executive Officers of the Company are as follows:

(c) Identification of Certain Significant Employees. None.

(d) Family Relationships. No Directors or Executive Officers are related to any other either by blood, marriage, or adoption.

(e) Business Experience. Information with respect to Directors is set forth under the heading “Election of Directors” in the definitive 2005 Proxy Statement, which by this reference is incorporated herein. All Executive Officers have held responsible positions with the Company for at least five years as described in (b) above.

(f) Involvement in Certain Legal Proceedings. None.

(g) Promoters and Control Persons. None.

(h) Audit Committee Financial Expert. Information with respect to the financial expert of the Board of Directors’ audit committee is set forth under the heading “Committees of the Board” in the definitive 2005 Proxy Statement, which by this reference is incorporated herein.

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(i) Identification of the Audit Committee. Information with respect to the composition of the Board of Directors’ audit committee is set forth under the heading “Committees of the Board” in the definitive 2005 Proxy Statement, which by this reference is incorporated herein.

Section 16(a) Beneficial Ownership Reporting Compliance. Section 16(a) of the Securities Exchange Act of 1934 requires officers and directors, and persons who own more than ten percent of a registered class of equity securities, to file reports of ownership and changes in ownership with the SEC and the New York Stock Exchange. Officers, directors, and beneficial owners of more than ten percent of any class of equity securities are required by SEC regulation to furnish the Company with copies of all Section 16(a) forms they file.

The Company has adopted procedures to assist its directors and executive officers in complying with Section 16(a) of the Exchange Act, as amended, which includes assisting in the preparation of forms for filing. For 2004, all but three reports were timely filed. Purchases of Company common stock by Richard M. Gardner, Director, consisting of 467 shares on August 16, 2004 and 475 shares on November 3, 2004, were reported on December 2, 2004. A sale of Company common stock by Thomas J Armstrong, Senior Vice President/Gas Resources & Energy Services, consisting of 341 shares on October 8, 2004, was reported on November 11, 2004.

Code of Business Conduct and Ethics. The Company has adopted a code of business conduct and ethics for its employees, including its chief executive officer, chief financial officer, chief accounting officer, and non-employee directors. A code of ethics is defined as written standards that are reasonably designed to deter wrongdoing and to promote: 1) honest and ethical conduct; 2) full, fair, accurate, timely, and understandable disclosure in reports and documents that a registrant files; 3) compliance with applicable governmental laws, rules, and regulations; 4) the prompt internal reporting of violations of the code to an appropriate person or persons identified in the code; and 5) accountability for adherence to the code. The Company’s Code of Business Conduct & Ethics can be viewed on the Company’s website (www.swgas.com). If any substantive amendments to the Code of Business Conduct & Ethics are made or any waivers are granted, including any implicit waiver, from a provision of the Code of Business Conduct & Ethics, to the Company’s chief executive officer, chief financial officer and chief accounting officer, the Company will disclose the nature of such amendment or waiver on the Company’s website, www.swgas.com.

Item 11. EXECUTIVE COMPENSATION

Information with respect to executive compensation is set forth under the heading “Executive Compensation and Benefits” in the definitive 2005 Proxy Statement, which by this reference is incorporated herein.

(a) Security Ownership of Certain Beneficial Owners. Information with respect to security ownership of certain beneficial owners is set forth under the heading “Securities Ownership by Directors, Director Nominees, Executive Officers, and Certain Beneficial Owners” in the definitive 2005 Proxy Statement, which by this reference is incorporated herein.

(b) Security Ownership of Management. Information with respect to security ownership of management is set forth under the heading “Securities Ownership by Directors, Director Nominees, Executive Officers, and Certain Beneficial Owners” in the definitive 2005 Proxy Statement, which by this reference is incorporated herein.

(c) Changes in Control. None.

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(d) Securities Authorized for Issuance Under Equity Compensation Plans.

At December 31, 2004, the Company had two stock-based compensation plans. With respect to the first plan, the Company may grant options to purchase shares of common stock to key employees and outside directors.

Pursuant to the terms of the management incentive plan, the Company may issue restricted stock in the form of performance shares to encourage key employees to remain in its employment to achieve short-term and long-term performance goals.

Additional information regarding the two equity compensation plans is included in Note 9 of the Notes to Consolidated Financial Statements in the 2004 Annual Report to Shareholders.

Item 13. CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS

None.

Item 14. PRINCIPAL ACCOUNTANT FEES AND SERVICES

Information with respect to accounting fees and services associated with PricewaterhouseCoopers LLP is set forth under the heading “Selection of Independent Accountants” in the definitive 2005 Proxy Statement, which by this reference is incorporated herein.

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PART IV

Item 15. EXHIBITS AND FINANCIAL STATEMENT SCHEDULES

(a) The following documents are filed as part of this report on Form 10-K:

(b) See LIST OF EXHIBITS.

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LIST OF EXHIBITS

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†Compensation Plans

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SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: March 14, 2005

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, this report has been signed below by the following persons on behalf of the Registrant and in the capacities and on the dates indicated.

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EXHIBIT INDEX

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