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                                                                    EXHIBIT 5.1




(214) 220-7700                                                  (214) 999-7700


                                September 5, 1997


Pioneer Natural Resources Company
1400 Williams Square West
5205 North O'Connor Boulevard
Irving, Texas  75039

Ladies and Gentlemen:

         We have acted as counsel for Pioneer Natural Resources Company, a
Delaware corporation (the "COMPANY"), in connection with the Company's
registration under the Securities Act of 1933, as amended (the "ACT"), of
3,553,033 shares of common stock, par value $0.01 per share (the "SHARES"), of
the Company pursuant to the Company's Registration Statement on Form S-8 (the
"REGISTRATION STATEMENT") filed with the Securities and Exchange Commission
(the "COMMISSION") on September 5, 1997.

         In reaching the opinions set forth herein, we have examined and are
familiar with originals or copies, certified or otherwise identified to our
satisfaction, of such documents and records of the Company and such statutes,
regulations and other instruments as we deemed necessary or advisable for
purposes of this opinion, including (i) the Registration Statement, (ii) the
Restated Certificate of Incorporation of the Company, as filed with the
Secretary of State of the State of Delaware, (iii) the Bylaws of the Company,
and (iv) certain minutes of meetings of, and resolutions adopted by, the Board
of Directors of the Company.

         We have assumed that (i) all information contained in all documents we
reviewed is true, correct and complete, (ii) all signatures on all documents we
reviewed are genuine, (iii) all documents submitted to us as originals are true
and complete, (iv) all documents submitted to us as copies are true and
complete copies of the originals thereof, and (v) all persons executing and
delivering the documents we examined were competent to execute and deliver such
documents. In addition, we have assumed that, upon exercise of the stock
options pursuant to which the Shares will be issued (the "OPTIONS"), (i) the
Shares will be issued in accordance with the Company's Long- Term Incentive
Plan (the "LONG-TERM INCENTIVE PLAN"), (ii) the full consideration for each
Share shall be paid to the Company and in no event will be less than the par
value for each Share, and (iii)

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Pioneer Natural Resources Company
September 5, 1997
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certificates evidencing the Shares will be properly executed and delivered by
the Company in accordance with the Delaware General Corporation Law (the
"DGCL").

         Based on the foregoing, and having due regard for the legal
considerations we deem relevant, we are of the opinion that the Shares, when
issued by the Company upon exercise of the Options in accordance with the
Long-Term Incentive Plan, will be legally issued, fully paid and
non-assessable.

         This opinion is limited in all respects to the laws of the State of
Texas, the DGCL and the federal laws of the United States of America. You
should be aware that we are not admitted to the practice of law in the State of
Delaware.

         This opinion letter may be filed as an exhibit to the Registration
Statement. In giving this consent, we do not thereby admit that we come within
the category of persons whose consent is required under Section 7 of the Act or
the rules and regulations of the Commission promulgated thereunder.

                                        Very truly yours,

                                        /s/ Vinson & Elkins L.L.P.