June 20, 2011 – Frontier Oil Corporation (NYSE: FTO) (“Frontier”) today announced that it has submitted written notice to the New York Stock Exchange (the “NYSE”) of its intention to voluntarily delist its common stock, no par value, from the NYSE pending successful closing of the merger of Frontier with Holly Corporation (the “Closing”). Pending the Closing, Frontier intends to have the NYSE file a Form 25 with the Securities and Exchange Commission (the “SEC”) on July 1, 2011 to delist its common stock. Also pending the Closing, Frontier intends to file a Form 15 with the SEC on or about July 11, 2011 in order to terminate the registration of the common stock under Section 12 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and to notify the SEC of the automatic suspension of its public reporting obligations under Sections 13(a) and 15(d) of the Exchange Act.

As previously announced, Frontier entered into an Agreement and Plan of Merger, dated as of February 21, 2011 (the “Merger Agreement”), by and among Frontier, Holly Corporation (“Holly”), and North Acquisition, Inc. (“Merger Sub”) pursuant to which Merger Sub will merge with and into Frontier (the “Merger”). Pursuant to the Merger Agreement, upon consummation of the Merger (a) each outstanding share of Frontier common stock (subject to certain exceptions) will be converted into 0.4811 shares of Holly common stock, (b) certain outstanding Frontier equity awards issued under the Frontier Omnibus Incentive Compensation Plan (as amended and as may be further amended from time to time, the “Frontier Plan”) will vest and be converted into shares of Holly common stock and (c) certain outstanding Frontier equity awards issued under the Frontier Plan will be converted into comparable Holly equity awards. As a result of the Merger, a trading market for Frontier’s common stock will no longer exist. In addition, upon the delisting, Frontier will qualify to terminate the registration of its common stock under Section 12 of the Exchange Act and exit the SEC periodic reporting system. The common stock will not be listed or registered on another national securities exchange or for quotation in a quotation medium.

As of the date of the filing of the Form 15, Frontier’s obligation to file certain reports under the Exchange Act, including Forms 10-K, 10-Q and 8-K, will be immediately suspended, and other filing requirements will terminate upon the effectiveness of the deregistration, which should occur 90 days after the filing of the Form 15.

About Frontier

Frontier Oil Corporation operates a 135,000 bpd refinery located in El Dorado, Kansas, and a 52,000 bpd refinery located in Cheyenne, Wyoming, and markets its refined products principally along the eastern slope of the Rocky Mountains and in other neighboring plains states. Information about the Company may be found on its website www.frontieroil.com.

Important Information for Investors and Shareholders

This communication does not constitute an offer to sell or the solicitation of an offer to buy any securities or a solicitation of any vote or approval. The issuance of Holly Corporation (“Holly”) common stock in connection with the proposed merger has been submitted to Holly’s stockholders for their consideration, and the proposed merger has been submitted to shareholders of Frontier Oil Corporation (“Frontier”) for their consideration. Holly has filed with the Securities and Exchange Commission (“SEC”) a registration statement on Form S-4 that includes a joint proxy statement to be used by Holly and Frontier to solicit the required approval of their shareholders in connection with the proposed merger and constituted a prospectus of Holly, which the SEC has declared effective. Holly and Frontier may also file other documents with the SEC concerning the proposed merger. INVESTORS AND SECURITY HOLDERS OF HOLLY AND FRONTIER ARE URGED TO READ THE JOINT PROXY STATEMENT/PROSPECTUS REGARDING THE PROPOSED MERGER AND OTHER RELEVANT DOCUMENTS FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY BECAUSE THEY CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED MERGER. Investors and security holders may obtain a free copy of the joint proxy statement/prospectus and other documents containing important information about Holly and Frontier through the website maintained by the SEC at www.sec.gov. Copies of the documents filed with the SEC by Holly are available free of charge on Holly’s website at www.hollycorp.com under the tab “Investors” or by contacting Holly’s Investor Relations Department at (214) 871-3555. Copies of documents filed with the SEC by Frontier are available free of charge on Frontier’s website at www.frontieroil.com under the tab “Investor Relations” and then under the tab “SEC Filings” or by contacting Frontier’s Investor Relations Department at (713) 688-9600.

Holly, Frontier and certain of their respective directors and executive officers may be deemed to be participants in the solicitation of proxies from the stockholders of Holly and shareholders of Frontier in connection with the proposed transaction. Information about the directors and executive officers of Holly is set forth in its proxy statement for its 2011 annual meeting of stockholders, which was filed with the SEC on March 31, 2011. Information about the directors and executive officers of Frontier is set forth in its proxy statement for its 2011 annual meeting of shareholders, which was filed with the SEC on March 21, 2011. These documents can be obtained free of charge from the sources indicated above. Other information regarding the participants in the proxy solicitation and a description of their direct and indirect interests, by security holdings or otherwise, are contained in the joint proxy statement/prospectus and other relevant materials filed with the SEC.

Cautionary Statement Regarding Forward-Looking Statements

This communication contains “forward-looking” statements within the meaning of Section 27A of the Securities Act of 1933, and Section 21E of the Securities Exchange Act of 1934. These include statements regarding the effects of the proposed merger and statements preceded by, followed by or that otherwise include the words “believes,” “expects,” “anticipates,” “intends,” “estimates,” or similar expressions. Forward-looking statements relating to expectations about future results or events are based upon information available to Holly and Frontier as of today’s date, and are not guarantees of the future performance of Holly, Frontier or the combined company, and actual results may vary materially from the results and expectations discussed. For instance, there is no assurance that the proposed merger will be consummated. The merger agreement will terminate if the companies do not receive the necessary approval of Holly’s stockholders or Frontier’s shareholders or government approvals or if either Holly or Frontier fails to satisfy conditions to closing. Additional risks and uncertainties related to the proposed merger include, but are not limited to, the successful integration of Holly’s and Frontier’s businesses and the combined company’s ability to compete in the highly competitive refining and marketing industry. The revenues, earnings and business prospects of Holly, Frontier and the combined company and their ability to achieve planned business objectives will be subject to a number of risks and uncertainties. These risks and uncertainties include, among other things, risks and uncertainties with respect to the actions of actual or potential competitive suppliers of refined petroleum products in Holly’s, Frontier’s and the combined company’s markets; the demand for and supply of crude oil and refined products; the spread between market prices for refined products and market prices for crude oil; the possibility of constraints on the transportation of refined products; the possibility of inefficiencies, curtailments or shutdowns in refinery operations or pipelines; effects of governmental and environmental regulations and policies; the availability and cost of financing; the effectiveness of capital investments and marketing strategies; efficiency in carrying out construction projects; the ability to acquire refined product operations or pipeline and terminal operations on acceptable terms and to integrate any existing or future acquired operations; the possibility of terrorist attacks and the consequences of any such attacks; and general economic conditions.

Holly and Frontier caution that the foregoing list of risks and uncertainties is not exclusive. Additional information concerning these and other risks is contained in Holly’s and Frontier’s most recently filed Annual Report on Form 10-K, subsequent Quarterly Report on Form 10-Q, recent Current Reports on Form 8-K and other SEC filings. All subsequent written and oral forward-looking statements concerning Holly, Frontier, the proposed merger or other matters and attributable to Holly or Frontier or any person acting on their behalf are expressly qualified in their entirety by the cautionary statements above. Neither Holly nor Frontier undertake any obligation to publicly update any of these forward-looking statements to reflect events or circumstances that may arise after the date hereof.

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