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                                    Filed by Pioneer Natural Resources USA, Inc.
                           Pursuant to Rule 425 under the Securities Act of 1933
                                    Subject Company: Parker & Parsley 84-A, Ltd.
                                                   Commission File No. 333-59094


                [Pioneer Natural Resources USA, Inc. Letterhead]



July 11, 2001




To the limited partners of Parker & Parsley 84-A,

         You may have recently received an offer from Sierra Fund 3 to purchase
your limited partnership interests for cash on a first-received, first-buy
basis, limited to a number of units such that Sierra Fund 3 will own no more
than 4.9% of the outstanding units.

         While Pioneer Natural Resources USA, Inc., as the general partner of
your limited partnership, is taking a neutral position on whether you should
accept or reject Sierra Fund 3's tender offer, we do believe that the offer is
undervalued compared to the fair value of your interests. We base our belief
primarily on the fact that we and our parent company, Pioneer Natural Resources
Company, are currently proposing an agreement and plan of merger for your
partnership that is based on a higher value for your limited partnership
interests than Sierra Fund 3 is offering you.

         You should consider the following as you evaluate Sierra Fund 3's
offer:

         o        Sierra Fund 3 is offering you $110.00 per partnership unit
                  payable in cash.

         o        The Pioneer Natural Resources Company offer pursuant to the
                  currently proposed merger would pay you $201.43 per
                  partnership unit using shares of its common stock, which are
                  traded on the New York Stock Exchange. Our view of the value
                  of the Pioneer Natural Resources Company common stock compared
                  to the cash offered by Sierra Fund 3 may be affected by our
                  conflict of interest, since we are participants in and
                  proponents of the proposed merger.

         o        Sierra Fund 3's offer is limited in number and appears not to
                  be available pro-rata to those who wish to sell, but on a
                  first-come, first-served basis. The proposed merger, on the
                  other hand, will give all limited partners of a participating
                  partnership the same treatment.

         o        It is uncertain whether and when our proposed merger will be
                  completed. The merger requires the approval of limited
                  partners, which may not be received. We will not be able to
                  request that approval until after a registration statement
                  relating to our parent company's common stock is declared
                  effective. No solicitation will be made using preliminary
                  materials. The proposed merger will not happen if the
                  registration statement is not declared effective or if we or
                  our parent company decide to abandon the proposed merger.
                  COPIES OF THE PRELIMINARY PROXY STATEMENT/PROSPECTUS MAY BE
                  OBTAINED WITHOUT CHARGE UPON



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                  REQUEST FROM PIONEER NATURAL RESOURCES COMPANY, 5205 NORTH
                  O'CONNOR BLVD., SUITE 1400, IRVING, TEXAS 75039, ATTENTION:
                  INVESTOR RELATIONS.

         A registration statement containing a preliminary proxy
statement/prospectus relating to the shares of our parent company's common stock
proposed to be issued in the merger has been filed with the Securities and
Exchange Commission, but has not yet become effective. Those securities may not
be sold nor may offers to buy be accepted prior to the time the registration
statement becomes effective. This letter shall not constitute an offer to sell
or the solicitation of an offer to buy nor shall there be any sale of those
securities in any State in which that offer, solicitation or sale would be
unlawful prior to registration or qualification under the securities laws of any
such State.

         INVESTORS ARE URGED TO READ THE PROXY STATEMENT/PROSPECTUS OF OUR
PARENT COMPANY, PIONEER NATURAL RESOURCES COMPANY, FILED WITH THE SEC,
ESPECIALLY THE FINAL FORM WHEN IT IS AVAILABLE, BECAUSE THE PROXY
STATEMENT/PROSPECTUS CONTAINS IMPORTANT INFORMATION ABOUT THE PROPOSED MERGER,
INCLUDING INFORMATION ABOUT:

         o        THE DIRECT AND INDIRECT INTERESTS OF US AND OUR PARENT COMPANY
                  AS PARTICIPANTS IN THE MERGER,

         o        OUR OWNERSHIP OF INTERESTS IN THE LIMITED PARTNERSHIPS,

         o        PRIOR CONTACTS BETWEEN US AND SIERRA FUND 3 REGARDING YOUR
                  LIMITED PARTNERSHIPS AND OTHER LIMITED PARTNERSHIPS,

         o        OUR CONFLICTING INTERESTS IN RECOMMENDING THE MERGER.

         INVESTORS MAY ALSO OBTAIN THE PRELIMINARY AND (WHEN FILED) FINAL PROXY
STATEMENT/PROSPECTUS AND OTHER RELEVANT DOCUMENTS RELATING TO THE PROPOSED
MERGER FREE THROUGH THE INTERNET WEB SITE THAT THE SEC MAINTAINS AT WWW.SEC.GOV.

                         Very truly yours,

                         PIONEER NATURAL RESOURCES USA, INC.



                         /s/ Mark Withrow

                         Mark Withrow
                         Executive Vice President, General Counsel and Secretary