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 the 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.

November 8, 2005

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