EXHIBIT 31.1

                      CHIEF EXECUTIVE OFFICER CERTIFICATION

I, Scott D. Sheffield, certify that:

1.   I have  reviewed  this  quarterly  report on Form 10-Q of  Pioneer  Natural
     Resources Company;

2.   Based on my knowledge, this report does not contain any untrue statement of
     a material  fact or omit to state a  material  fact  necessary  to make the
     statements made, in light of circumstances under which such statements were
     made, not misleading with respect to the period covered by this report;

3.   Based on my  knowledge,  the  financial  statements,  and  other  financial
     information  included  in  this  report,  fairly  present  in all  material
     respects the financial  condition,  results of operations and cash flows of
     the registrant as of, and for, the periods presented in this report;

4.   The  registrant's  other  certifying  officer  and  I are  responsible  for
     establishing and maintaining disclosure controls and procedures (as defined
     in Exchange Act Rules  13a-15(e) and 15d-15(e))  and internal  control over
     financial  reporting  (as  defined  in  Exchange  Act Rules  13a-15(f)  and
     15d-15(f)) for the registrant and have:

     (a)   Designed such  disclosure  controls  and  procedures,  or caused such
           disclosure  controls   and  procedures  to  be   designed  under  our
           supervision,  to ensure  that  material  information  relating to the
           registrant, including its consolidated subsidiaries, is made known to
           us by others within those entities, particularly during the period in
           which this report is being prepared;

     (b)   Designed  such internal  control over financial reporting,  or caused
           such internal control over financial  reporting to be  designed under
           our  supervision,  to  provide  reasonable  assurance  regarding  the
           reliability of financial  reporting and the preparation of  financial
           statements  for  external  purposes  in   accordance  with  generally
           accepted accounting principles;

     (c)   Evaluated the  effectiveness of the  registrant's disclosure controls
           and procedures and presented in this report our conclusions about the
           effectiveness of  the disclosure controls  and procedures,  as of the
           end of the  period covered  by this  report based on such evaluation;
           and

     (d)   Disclosed in  this report  any change  in the  registrant's  internal
           control   over  financial   reporting   that  occurred   during   the
           registrant's  most recent  fiscal quarter  (the  registrant's  fourth
           fiscal quarter in the  case of an annual report) that has  materially
           affected,  or  is  reasonably  likely  to  materially   affect,   the
           registrant's internal control over financial reporting; and

5.   The registrant's  other certifying  officer and I have disclosed,  based on
     our most recent evaluation of internal control over financial reporting, to
     the registrant's auditors and the audit committee of the registrant's board
     of directors (or persons performing the equivalent functions):

     (a)   All significant deficiencies and material weaknesses in the design or
           operation  of internal  control over  financial  reporting  which are
           reasonably likely  to adversely  affect the  registrant's  ability to
           record, process, summarize and report financial information; and

     (b)   Any fraud, whether or not material, that involves management or other
           employees who  have a  significant role in  the registrant's internal
           control over financial reporting.

August 7, 2006
                             /s/ Scott D. Sheffield
                             ----------------------------------------
                             Scott D. Sheffield, Chairman and
                             Chief Executive Officer