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                                                                     EXHIBIT 8.1

                             VINSON & ELKINS L.L.P.
                                Attorneys at Law
                           3700 Trammell Crow Center
                                2001 Ross Avenue
                            Dallas, Texas 75201-2975
                            Telephone (214) 220-7700
                            Fax       (214) 220-7716


September 20, 2001

Pioneer Natural Resources Company
5205 North O'Connor Blvd., Suite 1400
Irving, Texas 75039

Ladies and Gentlemen:

     We participated in the preparation of Registration Statement No. 333-59094
on Form S-4 filed with the Securities and Exchange Commission by Pioneer Natural
Resources Company, with respect to the merger of certain limited partnerships
with and into Pioneer Natural Resources USA, Inc. (as amended from time to time
and together with exhibits thereto, the "Registration Statement"), including the
discussion set forth in the Registration Statement under the heading "Material
U.S. Federal Income Tax Consequences." The discussion and the legal conclusions
with respect to United States federal income tax matters set forth therein
constitute our opinion, and we believe they are accurate and complete in all
material respects.

     Our opinion is rendered as of the time the Registration Statement is
declared effective and is based and conditioned upon the initial and continuing
accuracy of the facts and assumptions set forth in the Registration Statement.
Our opinion is also based upon provisions of the United States Internal Revenue
Code of 1986, as amended, regulations promulgated or proposed thereunder and
interpretations thereof by the Internal Revenue Service and the courts, all as
of the date of the Registration Statement, all of which are subject to change
with prospective or retroactive effect, and our opinion could be adversely
affected or rendered obsolete by any such change.

     We hereby consent to the use of our name in the Registration Statement and
to the filing of this opinion as part of the Registration Statement. This
consent does not constitute an admission that we are "experts" within the
meaning of such term as used in the United States Securities Act of 1933.


                                          Very truly yours,

                                          /s/ VINSON & ELKINS L.L.P.