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                                                                     EXHIBIT 8.1


                                 [W. John Glancy
                                   Letterhead]

   
                                  May 18, 1998
    


Giant Industries, Inc.
23733 North Scottsdale Road
Scottsdale, Arizona  85255

Holly Corporation
100 Crescent Court
Suite 1600
Dallas, Texas  75201

Ladies & Gentlemen:

         You have requested my opinion regarding certain federal income tax
consequences of the merger (the "Merger") of Holly Corporation ("Holly"), a
Delaware corporation, with and into Giant Industries, Inc. ("Giant"), a Delaware
corporation.

   
         In formulating this opinion, I have examined such documents as I deemed
appropriate, including the Agreement and Plan of Merger between Giant and Holly
dated as of April 14, 1998 (the "Merger Agreement") and the Joint Proxy
Statement/Prospectus (the "Joint Proxy Statement/Prospectus") included in
Amendment No. 1 to the Registration Statement on Form S-4, No. 333-51785, as
filed by Giant with the Securities and Exchange Commission on May 18, 1998 (the
"Registration Statement"). In addition, I have obtained such additional
information as I have deemed relevant and necessary through consultation with
various officers and representatives of Giant and Holly.
    

         The opinions set forth below assume (1) the accuracy of the statements
and facts concerning the Merger set forth in the Merger Agreement, the Joint
Proxy Statement/Prospectus, and the Registration Statement, (2) the consummation
of the Merger in the manner contemplated by, and in accordance with the terms
set forth in, the Merger Agreement, the Joint Proxy Statement/Prospectus and the
Registration Statement and (3) the accuracy of the representations made by Giant
and by Holly set forth in certificates delivered by Giant and by Holly dated the
date hereof.

         Based upon the facts and statements set forth above, my examination and
review of the documents referred to above and subject to the assumptions set
forth above, I am of the opinion that for federal income tax purposes:

                  1. The Merger will constitute a reorganization within the
                  meaning of Section 368(a) of the Internal Revenue Code of
                  1986, as amended (the "Code").
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Giant Industries, Inc.
Holly Corporation
May 18, 1998
Page 2


                  2. Giant and Holly will each be a party to the reorganization
                  within the meaning of Section 368(b) of the Code.

                  3. No gain or loss will be recognized by the Holly
                  stockholders upon their exchange of Holly common stock for
                  Giant common stock in the Merger, with possible exceptions to
                  the extent that such a stockholder receives cash as a portion
                  of the Merger Consideration as provided in the Merger
                  Agreement or in lieu of a fractional share.

         In addition, in my opinion, based on the facts and assumptions stated
above, the discussion set forth under the heading "Certain Federal Income Tax
Consequences" in the Joint Proxy Statement/Prospectus included in the
Registration Statement accurately describes, subject to the limitations and
qualifications set forth therein, the material federal income tax consequences
of the Merger for Giant, Holly, the stockholders of Giant, and the stockholders
of Holly who receive Merger Consideration pursuant to the terms of the Merger
Agreement.

         I express no opinion concerning any tax consequences of the Merger
other than as specifically set forth herein.

         The opinions set forth above are based on current provisions of the
Code, the Treasury Regulations promulgated thereunder, published pronouncements
of the Internal Revenue Service and case law, any of which may be changed at any
time with retroactive effect. Any change in applicable laws or facts and
circumstances surrounding the Merger, or any inaccuracy in the statements,
facts, assumptions and representations on which I have relied, may affect the
continuing validity of the opinions set forth herein. I assume no responsibility
to inform you of any such change or inaccuracy that may occur or come to my
attention.

         This opinion is rendered solely for your benefit in connection with the
transaction described above. This opinion may not be used or relied upon by any
other person and may not be disclosed, quoted, filed with a governmental agency
or otherwise referred to without my prior written consent.

         I hereby consent to the use of this opinion as an exhibit to the
Registration Statement and to the reference to me under the caption "Legal
Matters" in the Joint Proxy Statement/Prospectus.

                                                 Very truly yours,

   
                                                 /s/ W. John Glancy
                                                 W. John Glancy