UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549

                                    FORM 10-K

                  Annual Report Pursuant to Section 13 or 15(d)
                     of the Securities Exchange Act of 1934

                   For the fiscal year ended December 31, 1999


                         Commission File No. 33-38582-01

                           PARKER & PARSLEY 91-A, L.P.
             (Exact name of Registrant as specified in its charter)

                         Delaware                            75-2387572
       --------------------------------------           ---------------------
          (State or other jurisdiction of                  (I.R.S. Employer
          incorporation or organization)                Identification Number)

1400 Williams Square West, 5205 N. O'Connor Blvd., Irving, Texas       75039
----------------------------------------------------------------     ----------
            (Address of principal executive offices)                 (Zip code)

       Registrant's Telephone Number, including area code : (972) 444-9001

                 303 West Wall, Suite 101, Midland, Texas 79701
         (Former name, former address and former fiscal year, if changed
                               since last report)

        Securities registered pursuant to Section 12(b) of the Act: None

           Securities registered pursuant to Section 12(g) of the Act:
                 Limited partnership interests ($1,000 per unit)

Indicate by check mark whether the Registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during
the  preceding 12 months (or for such  shorter  period that the  Registrant  was
required  to file  such  reports),  and  (2) has  been  subject  to such  filing
requirements for the past 90 days. Yes / x / No / /

Indicate by check mark if disclosure of delinquent  filers  pursuant to Item 405
of Regulation  S-K is not contained  herein,  and will not be contained,  to the
best of Registrant's  knowledge,  in definitive proxy or information  statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. / x /

No  market  currently  exists  for  the  limited  partnership  interests  of the
Registrant.  Based on original  purchase  price the  aggregate  market  value of
limited  partnership  interests  owned by  non-affiliates  of the  Registrant is
$11,575,000.

As of March 8, 2000, the number of outstanding limited partnership interests was
11,620.

 The following documents are incorporated by reference into the indicated parts
                    of this Annual Report on Form 10-K: None


<PAGE>



Parts I and II of this Report contain  forward  looking  statements that involve
risks and uncertainties. Accordingly, no assurances can be given that the actual
events and results will not be materially different than the anticipated results
described  in the  forward  looking  statements.  See "Item 1.  Business"  for a
description of various factors that could  materially  affect the ability of the
Partnership to achieve the anticipated  results described in the forward looking
statements.

                                     PART I

ITEM 1.     Business

Parker &  Parsley  91-A,  L.P.  (the  "Partnership")  is a  limited  partnership
organized in 1991 under the laws of the State of Delaware. As of August 8, 1997,
Pioneer Natural Resources USA, Inc.  ("Pioneer USA") became the managing general
partner of the Partnership.  Prior to August 8, 1997, the Partnership's managing
general  partner  was  Parker  &  Parsley  Petroleum  USA,  Inc.  ("PPUSA"),   a
wholly-owned  subsidiary  of  Parker &  Parsley  Petroleum  Company  ("Parker  &
Parsley").  On August 7, 1997,  Parker & Parsley and Mesa Inc. ("Mesa") received
shareholder  approval  to merge and create  Pioneer  Natural  Resources  Company
("Pioneer").  On August 8, 1997,  PPUSA was merged with and into  Pioneer USA, a
wholly-owned  subsidiary  of  Pioneer,  resulting  in Pioneer USA  becoming  the
managing general partner of the Partnership as PPUSA's successor by merger.

A Registration  Statement,  as amended,  filed pursuant to the Securities Act of
1933,  registering limited partnership  interests  aggregating  $105,000,000 and
$45,000,000  in general  partnership  interests in a series of Delaware  limited
partnerships  formed  under the  Parker &  Parsley  91-92  Development  Drilling
Program,  was declared  effective by the Securities  and Exchange  Commission on
June 25,  1991.  On  September  30,  1991,  the  offering of limited and general
partnership  interests in the Partnership,  the first  partnership  formed under
such statement, was closed, with interests aggregating $11,620,000 being sold to
705  subscribers  of which  $3,512,000  were sold to 199  subscribers as general
partner interests and $8,108,000 were sold to 506 subscribers as limited partner
interests.  The general  partners were converted to limited partners on November
30, 1992.

The  Partnership  engages in oil and gas  development  and production and is not
involved in any industry  segment  other than oil and gas. See "Item 6. Selected
Financial Data" for a summary of the Partnership's oil and gas sales, net income
and identifiable assets.

The principal  markets during 1999 for the oil produced by the Partnership  were
refineries  and  oil  transmission  companies  that  have  facilities  near  the
Partnership's oil producing  properties.  During 1999,  Pioneer USA marketed the
Partnership's gas to a variety of purchasers, none of which accounted for 10% or
more of the Partnership's oil and gas revenues.  Of the Partnership's  total oil
and gas revenues for 1999,  approximately 36% and 17% were attributable to sales
made to Plains All American Inc. and TEPPCO Crude Oil LLC, respectively. Pioneer
USA is of the  opinion  that the  loss of any one  purchaser  would  not have an
adverse  effect on its ability to sell its oil and gas production or natural gas
products.

The Partnership's revenues,  profitability,  cash flow and future rate of growth
are highly dependent on the prevailing prices of oil and gas, which are affected
by  numerous  factors  beyond  the  Partnership's  control.  Oil and gas  prices
historically  have been very volatile.  A substantial or extended decline in the
prices of oil or gas  could have a material  adverse effect on the Partnership's

                                        2


<PAGE>



revenues,  profitability and cash flow and could,  under certain  circumstances,
result in a reduction in the  carrying  value of the  Partnership's  oil and gas
properties.

Federal and state  regulation of oil and gas operations  generally  includes the
fixing of maximum prices for regulated categories of natural gas, the imposition
of maximum  allowable  production rates, the taxation of income and other items,
and the protection of the  environment.  Although the Partnership  believes that
its business operations do not impair  environmental  quality and that its costs
of complying with any  applicable  environmental  regulations  are not currently
significant,   the  Partnership  cannot  predict  what,  if  any,  effect  these
environmental regulations may have on its current or future operations.

The Partnership  does not have any employees of its own. Pioneer USA employs 687
persons,  many of whom  dedicated  a part of their  time to the  conduct  of the
Partnership's business during the period for which this report is filed. Pioneer
USA is responsible for all management functions.

Numerous  uncertainties  exist in estimating  quantities of proved  reserves and
future net revenues  therefrom.  The  estimates  of proved  reserves and related
future net revenues  set forth in this report are based on various  assumptions,
which may ultimately  prove to be inaccurate.  Therefore,  such estimates should
not be construed as estimates of the current  market value of the  Partnership's
proved reserves.

No material part of the  Partnership's  business is seasonal and the Partnership
conducts no foreign operations.

ITEM 2.     Properties

The  Partnership's  properties  consist of leasehold  interests in properties on
which oil and gas wells are located.  Such property  interests are often subject
to landowner  royalties,  overriding  royalties  and other oil and gas leasehold
interests.

Fractional  working  interests in  developmental  oil and gas prospects  located
primarily  in the  Spraberry  Trend  Area of West  Texas  were  acquired  by the
Partnership,  resulting in the Partnership's participation in the drilling of 48
oil and gas wells.  One well has been  plugged and  abandoned.  At December  31,
1999, the Partnership had 47 wells producing.

For  information  relating  to the  Partnership's  estimated  proved oil and gas
reserves at December 31, 1999,  1998 and 1997,  see Note 7 of Notes to Financial
Statements  included in "Item 8. Financial  Statements and  Supplementary  Data"
below. Such reserves have been estimated by the engineering staff of Pioneer USA
with  a  review  by  Williamson  Petroleum  Consultants,  Inc.,  an  independent
petroleum consultant.

ITEM 3.     Legal Proceedings

The  Partnership  from  time to time is a party  to  various  legal  proceedings
incidental to its business  involving claims in oil and gas leases or interests,
other  claims for damages in amounts not in excess of 10% of its current  assets
and other  matters,  none of which  Pioneer  USA  believes to be material to the
Partnership.

                                        3


<PAGE>



ITEM 4.     Submission of Matters to a Vote of Security Holders

There were no matters  submitted to a vote of security holders during the fourth
quarter of 1999.

                                        4


<PAGE>



                                     PART II

ITEM 5.     Market for Partnership's Common Equity and Related Stockholder
              Matters

At March 8, 2000, the Partnership  had 11,620  outstanding  limited  partnership
interests  held of record by 722  subscribers.  There is no  established  public
trading  market  for  the  limited  partnership  interests.  Under  the  limited
partnership  agreement,  Pioneer USA has made  certain  commitments  to purchase
partnership interests at a computed value.

Revenues which, in the sole judgement of the managing general  partner,  are not
required to meet the  Partnership's  obligations are distributed to the partners
at least quarterly in accordance with the limited partnership agreement.  During
the years ended  December  31,  1999 and 1998,  distributions  of  $432,629  and
$427,238, respectively, were made to the limited partners.

ITEM 6.     Selected Financial Data

The  following  table sets forth  selected  financial  data for the years  ended
December 31:
<TABLE>
                                 1999         1998         1997         1996         1995
                              ----------   ----------   ----------   ----------   ----------
<S>                           <C>          <C>          <C>          <C>          <C>
Operating results:
  Oil and gas sales           $1,140,335   $  955,645   $1,411,247   $1,629,975   $1,387,494
                               =========    =========    =========    =========    =========
  Impairment of oil and gas
    properties                $   66,898   $  306,043   $  485,158   $      -     $1,008,771
                               =========    =========    =========    =========    =========
  Net income (loss)           $  266,803   $ (280,631)  $   (7,029)  $  654,054   $ (755,419)
                               =========    =========    =========    =========    =========
  Allocation of net
   income (loss):
     Managing general partner $    2,668   $   (2,806)  $      (70)  $    6,540   $   (7,554)
                               =========    =========    =========    =========    =========
     Limited partners         $  264,135   $ (277,825)  $   (6,959)  $  647,514   $ (747,865)
                               =========    =========    =========    =========    =========
  Limited partners' net
    income (loss) per limited
    partnership interest      $    22.73   $   (23.91)  $     (.60)  $    55.72   $   (64.36)
                               =========    =========    =========    =========    =========
  Limited partners' cash
    distributions per limited
    partnership interest      $    37.23   $    36.77   $    75.11   $    70.97   $    64.70
                               =========    =========    =========    =========    =========
At year end:
  Identifiable assets         $2,531,606   $2,667,803   $3,402,546   $4,289,878   $4,511,078
                               =========    =========    =========    =========    =========
</TABLE>
                                        5


<PAGE>



ITEM 7.     Management's Discussion and Analysis of Financial Condition and
              Results of Operations

Results of Operations

1999 compared to 1998

The  Partnership's  1999 oil and gas revenues  increased 19% to $1,140,335  from
$955,645 in 1998.  The increase in revenues  resulted from higher average prices
received,  offset by a decline in  production.  In 1999,  44,203 barrels of oil,
20,617 barrels of natural gas liquids ("NGLs") and 100,615 mcf of gas were sold,
or 81,589 barrel of oil equivalents  ("BOEs").  In 1998,  49,403 barrels of oil,
21,220  barrels of NGLs and 108,617 mcf of gas were sold, or 88,726 BOEs. Due to
the  decline  characteristics  of the  Partnership's  oil  and  gas  properties,
management expects a certain amount of decline in production in the future until
the Partnership's economically recoverable reserves are fully depleted.

The average  price  received per barrel of oil  increased  $4.42,  or 34%,  from
$13.15 in 1998 to $17.57 in 1999.  The average price received per barrel of NGLs
increased  $2.90, or 45%, from $6.44 in 1998 to $9.34 in 1999. The average price
received per mcf of gas  increased  9% from $1.56 in 1998 to $1.70 in 1999.  The
market price for oil and gas has been extremely volatile in the past decade, and
management expects a certain amount of volatility to continue in the foreseeable
future.  The Partnership may therefore sell its future oil and gas production at
average prices lower or higher than that received in 1999.

The volatility of commodity prices has had, and continues to have, a significant
impact on the Partnership's revenues and operating cash flow and could result in
additional  decreases to the  carrying  value of the  Partnership's  oil and gas
properties.

A gain on  disposition  of  assets  of $1,096  recognized  during  1999 was from
equipment credits received on one temporarily abandoned property.

Total costs and expenses decreased in 1999 to $884,937 as compared to $1,247,302
in 1998, a decrease of $362,365, or 29%. The decrease was due to declines in the
impairment of oil and gas properties,  depletion and production costs, offset by
an increase in general and administrative expenses ("G&A").

Production  costs were  $514,994 in 1999 and  $535,948 in 1998,  resulting  in a
decrease of $20,954,  or 4%. This decrease  resulted from declines in ad valorem
taxes,  well  maintenance  costs and  workover  costs,  offset by an increase in
production taxes due to increased oil and gas revenues.

G&A's  components are independent  accounting and engineering  fees and managing
general  partner  personnel  and  operating  costs.   During  this  period,  G&A
increased,  in  aggregate,  72% from  $43,977 in 1998 to  $75,454  in 1999.  The
Partnership  paid the managing  general  partner  $34,110 in 1999 and $24,415 in
1998 for G&A incurred on behalf of the Partnership.  G&A is allocated,  in part,
to the  Partnership  by the managing  general  partner.  Allocated  expenses are
determined by the managing  general  partner based upon the level of activity of
the  Partnership  relative to the  non-partnership  activities  of the  managing
general partner.  The method of  allocation has  been  consistent  over the past

                                        6


<PAGE>



several  years with certain  modifications  incorporated  to reflect  changes in
Pioneer USA's overall business activities.

In  accordance  with  Statement  of  Financial  Accounting  Standards  No.  121,
"Accounting for the Impairment of Long-Lived Assets and for Long-Lived Assets to
be  Disposed  Of"  ("SFAS  121"),  the  managing  general  partner  reviews  the
Partnership's  oil  and  gas  properties  for  impairment   whenever  events  or
circumstances  indicate a decline in the recoverability of the carrying value of
the  Partnership's  assets  may have  occurred.  As a result of the  review  and
evaluation of its long-lived assets for impairment,  the Partnership  recognized
non-cash  charges of $66,898 and $306,043  related to its oil and gas properties
during 1999 and 1998, respectively.

Depletion  decreased  in 1999 to $227,591 as compared to $361,334 in 1998.  This
represented  a decrease of $133,743,  or 37%. This decrease was the result of an
increase  in proved  reserves  during  1999 due to higher  commodity  prices,  a
reduction  in the  Partnership's  net  depletable  basis from  charges  taken in
accordance  with SFAS 121 during the fourth quarter of 1998 and a decline in oil
production of 5,200  barrels for the period ended  December 31, 1999 compared to
the same period in 1998.

1998 compared to 1997

The  Partnership's  1998 oil and gas  revenues  decreased  32% to $955,645  from
$1,411,247 in 1997. The decrease in revenues  resulted from lower average prices
received. In 1998, 49,403 barrels of oil, 21,220 barrels of NGLs and 108,617 mcf
of gas were sold, or 88,726 BOEs. In 1997,  51,993 barrels of oil, 8,865 barrels
of NGLs and 135,829 mcf of gas were sold, or 83,496 BOEs.

Consistent with the managing general  partner,  the Partnership has historically
accounted for processed  natural gas production as wellhead  production on a wet
gas basis.  Effective  September  30, 1997, as a result of the merger with Mesa,
the managing  general partner  accounts for processed  natural gas production in
two  components:  natural gas  liquids  and dry residue  gas. As a result of the
change in the managing general  partner's  policy,  the Partnership now accounts
for processed  natural gas  production as processed  natural gas liquids and dry
residue gas.  Consequently,  separate product volumes will not be comparable for
periods prior to September 30, 1997.  Also,  prices for gas products will not be
comparable as the price per mcf for natural gas for the year ended  December 31,
1998 is the price received for dry residue gas and the price per mcf for natural
gas produced  prior to October 1997 was  presented as a price for wet gas (i.e.,
natural gas liquids combined with dry residue gas).

The average  price  received per barrel of oil  decreased  $6.40,  or 33%,  from
$19.55 in 1997 to $13.15 in 1998.  The average price received per barrel of NGLs
decreased $3.60, or 36%, from $10.04 in 1997 to $6.44 in 1998. The average price
received per mcf of gas decreased 31% from $2.25 in 1997 to $1.56 in 1998.

Total  costs  and  expenses  decreased  in 1998 to  $1,247,302  as  compared  to
$1,432,817  in 1997,  a decrease of  $185,515,  or 13%.  The decrease was due to
declines in the impairment of oil and gas properties,  production  costs and G&A
expenses, offset by an increase in depletion.

                                        7


<PAGE>



Production  costs were  $535,948 in 1998 and  $570,417 in 1997,  resulting  in a
decrease of $34,469,  or 6%. This decrease resulted from a decline in production
taxes  due to the  decline  in oil  and  gas  revenues  and a  decrease  in well
maintenance costs.

During this period,  G&A  decreased,  in aggregate,  12% from $49,938 in 1997 to
$43,977 in 1998. The  Partnership  paid the managing  general partner $24,415 in
1998 and $42,044 in 1997 for G&A incurred on behalf of the Partnership.

The  Partnership  recognized  non-cash SFAS 121 charges of $306,043 and $485,158
related to its oil and gas properties during 1998 and 1997, respectively.

Depletion  increased  in 1998 to $361,334 as compared to $327,304 in 1997.  This
represented  an increase of $34,030,  or 10%.  This increase was the result of a
decline in proved reserves during 1998 due to lower commodity prices,  offset by
a reduction in the  Partnership's  net  depletable  basis from charges  taken in
accordance  with SFAS 121 during the fourth  quarter of 1997 and a reduction  in
oil  production of 2,590 barrels for the period ended December 31, 1998 compared
to the same period in 1997.

Impact of inflation and changing prices on sales and net loss

Inflation  generally  does  not  impact  revenues  in the oil and gas  industry.
However,  inflation generally does impact expenses, the most significant for the
Partnership is lease operating expenses.

The petroleum  industry has been  characterized by volatile oil, NGL and natural
gas commodity prices and relatively stable supplier costs during the three years
ended  December  31,  1999.  During 1997 and 1998,  weather  patterns,  regional
economic  recessions and political matters combined to cause worldwide crude oil
supplies to exceed demand. As a result, crude oil prices declined  substantially
from the  price  levels of 1996.  Also  during  1997 and  1998,  but to a lesser
extent,  market  prices for natural gas  declined.  During  1999,  the price per
barrel for oil production similar to the Partnership's ranged from approximately
$11.00 to $24.00.  The  decrease in crude oil exports  during 1999 by members of
the Organization of Petroleum  Exporting  Countries ("OPEC") and other crude oil
exporting nations has resulted in higher Partnership revenues and operating cash
flow as compared to 1998.

Prices for natural gas are subject to ordinary seasonal  fluctuations,  and this
volatility of natural gas prices may result in production  being  curtailed and,
in some cases, wells being completely shut-in.

Liquidity and Capital Resources

Net Cash Provided by Operating Activities

Net cash provided by operating  activities  increased  $109,226  during the year
ended December 31, 1999 from 1998. The increase was primarily attributable to an
increase of $63,191 in oil and gas revenues and reductions in operating costs of
$20,441 that have resulted from the managing general  partner's cost containment
measures and a decline in G&A expenses paid of $25,594.

                                        8


<PAGE>



Net Cash Used in Investing Activities

The   Partnership's   investing   activities   during  1999  and  1998  included
expenditures related to equipment upgrades on various oil and gas properties.

Proceeds from asset  dispositions of $1,096  recognized  during 1999 were due to
equipment credits received on one temporarily  abandoned property.  During 1998,
proceeds  from asset  dispositions  of $5,617 were  recognized  due to equipment
credits received on active properties.

Net Cash Used in Financing Activities

In 1999, cash  distributions to the partners were $436,999,  of which $4,370 was
distributed  to the  managing  general  partner  and  $432,629  to  the  limited
partners.  In 1998, cash  distributions to the partners were $431,553,  of which
$4,315 was  distributed  to the  managing  general  partner and  $427,238 to the
limited partners.

Since  the  first  quarter  of 1999,  world  crude oil  prices  have  increased,
primarily as a result of decreases in crude oil supplies made  available by OPEC
and other crude oil exporting nations.  During the period from the third quarter
of 1997 through the first  quarter of 1999,  there was a  significant  declining
trend in world oil prices and, to a lesser  extent,  natural gas prices.  During
the first quarter of 1999,  OPEC and certain  other crude oil exporting  nations
announced  reductions  in their planned  export  volumes.  These  announcements,
together with the enactment of announced reductions in export volumes,  have had
a positive impact on world crude oil prices. No assurances can be given that the
reductions  in export  volumes or the  positive  trend in oil and gas  commodity
prices can be sustained for an extended period of time.

Proposal to acquire partnerships

On September 8, 1999,  Pioneer USA filed a preliminary  proxy statement with the
SEC  proposing  an  agreement  and plan of merger to the limited  partners of 25
publicly-held  Parker & Parsley  limited  partnerships.  The  preliminary  proxy
statement is non-binding and is subject to, among other things, consideration of
offers  from third  parties to  purchase  any  partnership  or its  assets,  the
majority approval of the limited partners in each partnership and the resolution
of SEC review comments. Pioneer is continuing to evaluate the feasibility of the
proposed  agreement and plan of merger;  however,  the current  commodity  price
outlook has diminished the  likelihood  that the proposed  agreement and plan of
merger will be consummated.

Year 2000 project readiness

As the year 2000 was  approaching,  the inability of some computer  programs and
embedded  technologies to distinguish between "1900" and "2000" gave rise to the
"Year 2000" problem.  Such computer programs and related technology were at risk
to fail outright or communicate  inaccurate data, if not remediated or replaced.
With the  proliferation  of electronic data  interchange,  the Year 2000 problem
represented a  significant  exposure to the entire  global  community,  the full
extent of which could not be accurately assessed prior to the year 2000.

                                        9


<PAGE>



In proactive  response to the Year 2000 problem,  the managing  general  partner
established a "Year 2000" project that  assessed,  to the extent  possible,  the
Partnership's  and the managing  general  partner's  internal Year 2000 problem;
took remedial  actions  necessary to minimize the Year 2000 risk exposure to the
managing  general  partner and  significant  third parties with whom it has data
interchange;  and, tested the managing general  partner's  systems and processes
once remedial actions were taken.  The managing general partner  contracted with
IBM Global  Services to perform the assessment  and remedial  phases of its Year
2000 project.  The managing  general  partner's  total costs related to the Year
2000 problem were $2.5 million.

The  managing  general  partner  has  closely   monitored  its  information  and
non-information   technology  systems  since  the  beginning  of  2000  and  has
identified no significant  Year 2000 failures or problems.  The managing general
partner  will  continue to monitor  Year 2000 risks and issues.  There can be no
assurances that unforeseen problems will not be encountered in the future.

                                       10


<PAGE>



ITEM 8.     Financial Statements and Supplementary Data

                          Index to Financial Statements

                                                                       Page

Financial Statements of Parker & Parsley 91-A, L.P.:
  Independent Auditors' Report - Ernst & Young LLP.................      12
  Independent Auditors' Report - KPMG LLP..........................      13
  Balance Sheets as of December 31, 1999 and 1998..................      14
  Statements of Operations for the Years Ended December 31,
    1999, 1998 and 1997............................................      15
  Statements of Partners' Capital for the Years Ended
    December 31, 1999, 1998 and 1997...............................      16
  Statements of Cash Flows for the Years Ended December 31,
    1999, 1998 and 1997............................................      17
  Notes to Financial Statements....................................      18



                                       11


<PAGE>



                          INDEPENDENT AUDITORS' REPORT

The Partners
Parker & Parsley 91-A, L.P.
  (A Delaware Limited Partnership):

We have audited the balance sheets of Parker & Parsley 91-A, L.P. as of December
31, 1999 and 1998, and the related  statements of operations,  partners' capital
and cash flows for the years then  ended.  These  financial  statements  are the
responsibility of the Partnership's management. Our responsibility is to express
an opinion on these financial statements based on our audits.

We conducted our audits in accordance with auditing standards generally accepted
in the United States. Those standards require that we plan and perform the audit
to obtain reasonable  assurance about whether the financial  statements are free
of material misstatement. An audit includes examining, on a test basis, evidence
supporting  the amounts and  disclosures in the financial  statements.  An audit
also includes assessing the accounting principles used and significant estimates
made by  management,  as well as  evaluating  the  overall  financial  statement
presentation.  We believe  that our audits  provide a  reasonable  basis for our
opinion.

In our opinion,  the financial  statements  referred to above present fairly, in
all material respects,  the financial position of Parker & Parsley 91-A, L.P. as
of December 31, 1999 and 1998,  and the results of its  operations  and its cash
flows  for the  years  then  ended  in  conformity  with  accounting  principles
generally accepted in the United States.

                                                  Ernst & Young LLP

Dallas, Texas
March 10, 2000

                                       12


<PAGE>




                          INDEPENDENT AUDITORS' REPORT

The Partners
Parker & Parsley 91-A, L.P.
  (A Delaware Limited Partnership):

We have audited the statement of operations, partners' capital and cash flows of
Parker &  Parsley  91-A,  L.P.  for the year  ended  December  31,  1997.  These
financial statements are the responsibility of the Partnership's management. Our
responsibility  is to express an opinion on these financial  statements based on
our audit.

We conducted our audit in accordance with generally accepted auditing standards.
Those standards  require that we plan and perform the audit to obtain reasonable
assurance   about  whether  the  financial   statements  are  free  of  material
misstatement.  An audit includes examining, on a test basis, evidence supporting
the amounts and disclosures in the financial statements.  An audit also includes
assessing the  accounting  principles  used and  significant  estimates  made by
management,  as well as evaluating the overall financial statement presentation.
We believe that our audit provides a reasonable basis for our opinion.

In our opinion,  the financial  statements referred to above presents fairly, in
all  material  respects,  the results of  operations  and cash flows of Parker &
Parsley 91-A,  L.P. for the year ended  December 31, 1997,  in  conformity  with
generally accepted accounting principles.

                                                  KPMG LLP


Midland, Texas
March 20, 1998

                                       13


<PAGE>



                           PARKER & PARSLEY 91-A, L.P.
                        (A Delaware Limited Partnership)

                                 BALANCE SHEETS
                                   December 31


                                                     1999             1998
                                                  -----------     -----------
                 ASSETS

Current assets:
  Cash                                            $   226,846     $   158,378
  Accounts receivable - oil and gas sales             177,988         111,524
                                                   ----------      ----------
          Total current assets                        404,834         269,902
                                                   ----------      ----------
Oil and gas properties - at cost, based on the
  successful efforts accounting method              9,701,521       9,678,161
Accumulated depletion                              (7,574,749)     (7,280,260)
                                                   ----------      ----------
          Net oil and gas properties                2,126,772       2,397,901
                                                   ----------      ----------
                                                  $ 2,531,606     $ 2,667,803
                                                   ==========      ==========
LIABILITIES AND PARTNERS' CAPITAL

Current liabilities:
  Accounts payable - affiliate                    $    48,061     $    14,062

Partners' capital:
  Managing general partner                             24,879          26,581
  Limited partners (11,620 interests)               2,458,666       2,627,160
                                                   ----------      ----------
                                                    2,483,545       2,653,741
                                                   ----------      ----------
                                                  $ 2,531,606     $ 2,667,803
                                                   ==========      ==========


   The accompanying notes are an integral part of these financial statements.

                                       14


<PAGE>



                           PARKER & PARSLEY 91-A, L.P.
                        (A Delaware Limited Partnership)

                            STATEMENTS OF OPERATIONS
                         For the years ended December 31




                                           1999          1998          1997
                                        ----------    ----------    ----------
Revenues:
  Oil and gas                           $1,140,335    $  955,645    $1,411,247
  Interest                                  10,309        11,026        14,541
  Gain on disposition of assets              1,096           -             -
                                         ---------     ---------     ---------
                                         1,151,740       966,671     1,425,788
                                         ---------     ---------     ---------
Costs and expenses:
  Oil and gas production                   514,994       535,948       570,417
  General and administrative                75,454        43,977        49,938
  Impairment of oil and gas properties      66,898       306,043       485,158
  Depletion                                227,591       361,334       327,304
                                         ---------     ---------     ---------
                                           884,937     1,247,302     1,432,817
                                         ---------     ---------     ---------
Net income (loss)                       $  266,803    $ (280,631)   $   (7,029)
                                         =========     =========     =========
Allocation of net income (loss):
  Managing general partner              $    2,668    $   (2,806)   $      (70)
                                         =========     =========     =========
  Limited partners                      $  264,135    $ (277,825)   $   (6,959)
                                         =========     =========     =========
Net income (loss) per limited
  partners' interest                    $    22.73    $   (23.91)   $     (.60)
                                         =========     =========     =========



   The accompanying notes are an integral part of these financial statements.

                                       15


<PAGE>



                           PARKER & PARSLEY 91-A, L.P.
                        (A Delaware Limited Partnership)

                         STATEMENTS OF PARTNERS' CAPITAL




                                         Managing
                                         general       Limited
                                         partner       partners       Total
                                        ----------    ----------    ----------

Partners' capital at January 1, 1997    $   42,588    $4,211,929    $4,254,517
     Distributions                          (8,816)     (872,747)     (881,563)
     Net loss                                  (70)       (6,959)       (7,029)
                                         ---------     ---------     ---------

Partners' capital at December 31, 1997      33,702     3,332,223     3,365,925
     Distributions                          (4,315)     (427,238)     (431,553)
     Net loss                               (2,806)     (277,825)     (280,631)
                                         ---------     ---------     ---------

Partners' capital at December 31, 1998      26,581     2,627,160     2,653,741
     Distributions                          (4,370)     (432,629)     (436,999)
     Net income                              2,668       264,135       266,803
                                         ---------     ---------     ---------

Partners' capital at December 31, 1999  $   24,879    $2,458,666    $2,483,545
                                         =========     =========     =========






   The accompanying notes are an integral part of these financial statements.

                                       16


<PAGE>



                           PARKER & PARSLEY 91-A, L.P.
                        (A Delaware Limited Partnership)

                            STATEMENTS OF CASH FLOWS
                         For the years ended December 31



                                                1999         1998        1997
                                              ---------   ---------   ----------
Cash flows from operations:
  Net income (loss)                           $ 266,803   $(280,631)  $  (7,029)
  Adjustment to reconcile net income (loss)
    to net cash provided by operating
    activities:
      Impairment of oil and gas properties       66,898     306,043     485,158
      Depletion                                 227,591     361,334     327,304
      Gain on disposition of assets              (1,096)        -           -
  Changes in assets and liabilities:
      Accounts receivable                       (66,464)     54,318      68,373
      Accounts payable                           33,999     (22,559)      1,260
                                               --------    --------    --------
        Net cash provided by operating
          activities                            527,731     418,505     875,066
                                               --------    --------    --------
Cash flows from investing activities:
  Additions to oil and gas properties           (23,360)    (15,294)    (14,946)
  Proceeds from asset disposition                 1,096       5,617         -
                                               --------    --------    --------
        Net cash used in investing activities   (22,264)     (9,677)    (14,946)
                                               --------    --------    --------
Cash flows used in financing activities:
  Cash distributions to partners               (436,999)   (431,553)   (881,563)
                                               --------    --------    --------
Net increase (decrease) in cash                  68,468     (22,725)    (21,443)
Cash at beginning of year                       158,378     181,103     202,546
                                               --------    --------    --------
Cash at end of year                           $ 226,846   $ 158,378   $ 181,103
                                               ========    ========    ========




   The accompanying notes are an integral part of these financial statements.

                                       17


<PAGE>



                           PARKER & PARSLEY 91-A, L.P.
                        (A Delaware Limited Partnership)

                          NOTES TO FINANCIAL STATEMENTS
                        December 31, 1999, 1998 and 1997

Note 1.     Organization and nature of operations

       Parker & Parsley 91-A, L.P. (the  "Partnership") is a limited partnership
organized in 1991 under the laws of the State of Delaware. As of August 8, 1997,
Pioneer Natural Resources USA, Inc.  ("Pioneer USA") became the managing general
partner of the Partnership.  Prior to August 8, 1997, the Partnership's managing
general  partner  was  Parker  &  Parsley  Petroleum  USA,  Inc.  ("PPUSA"),   a
wholly-owned  subsidiary  of  Parker &  Parsley  Petroleum  Company  ("Parker  &
Parsley").  On  August  7,  1997,  Parker  &  Parsley  and  Mesa  Inc.  received
shareholder  approval  to merge and create  Pioneer  Natural  Resources  Company
("Pioneer").  On August 8, 1997,  PPUSA was merged with and into  Pioneer USA, a
wholly-owned  subsidiary  of  Pioneer,  resulting  in Pioneer USA  becoming  the
managing general partner of the Partnership as PPUSA's successor by merger.

       The  Partnership  engages in oil and gas  development  and  production in
Texas and is not involved in any industry segment other than oil and gas.

Note 2.     Summary of significant accounting policies

       A summary of the significant  accounting policies consistently applied in
the preparation of the accompanying financial statements follows:

       Oil and gas properties - The Partnership  utilizes the successful efforts
method of accounting for its oil and gas  properties  and equipment.  Under this
method, all costs associated with productive wells and nonproductive development
wells are  capitalized  while  nonproductive  exploration  costs  are  expensed.
Capitalized  costs relating to proved properties are depleted using the unit-of-
production method on a property-by-property  basis based on proved oil (dominant
mineral)  reserves as  determined by the  engineering  staff of Pioneer USA, the
Partnership's  managing general partner,  and reviewed by independent  petroleum
consultants.  The carrying  amounts of properties sold or otherwise  disposed of
and the related  allowances for depletion are  eliminated  from the accounts and
any gain or loss is included in operations.

       Impairment  of  long-lived  assets  - In  accordance  with  Statement  of
Financial  Accounting  Standards  No. 121,  "Accounting  for the  Impairment  of
Long-Lived Assets and for Long-Lived Assets to be Disposed Of" ("SFAS 121"), the
Partnership  reviews its long-lived  assets to be held and used on an individual
property  basis,  including  oil and gas  properties  accounted  for  under  the
successful  efforts  method of  accounting,  whenever  events  or  circumstances
indicate  that the  carrying  value of those assets may not be  recoverable.  An
impairment  loss is indicated  if the sum of the  expected  future cash flows is
less  than  the  carrying  amount  of the  assets.  In  this  circumstance,  the
Partnership  recognizes an impairment  loss for the amount by which the carrying
amount of the asset exceeds the estimated fair value of the asset.

                                       18


<PAGE>



       Use of estimates in the preparation of financial statements - Preparation
of the accompanying  financial  statements in conformity with generally accepted
accounting principles requires management to make estimates and assumptions that
affect  the  reported  amounts  of assets  and  liabilities  and  disclosure  of
contingent  assets and  liabilities at the date of the financial  statements and
the  reporting  amounts of revenues and expenses  during the  reporting  period.
Actual results could differ from those estimates.

       Net  income  (loss)  per  limited  partnership  interest - The net income
(loss) per limited  partnership  interest is  calculated  by using the number of
outstanding limited partnership interests.

       Income taxes - A Federal  income tax  provision  has not been included in
the  financial  statements as the income of the  Partnership  is included in the
individual Federal income tax returns of the respective partners.

       Statements  of cash flows - For  purposes of reporting  cash flows,  cash
includes depository accounts held by banks.

       General and administrative expenses - General and administrative expenses
are allocated,  in part, to the  Partnership by the managing  general partner or
its  affiliates.  Allocated  expenses are  determined  by the  managing  general
partner  based upon the level of  activity  of the  Partnership  relative to the
non-partnership  activities  of the  managing  general  partner.  The  method of
allocation  has been  consistent  over  the  past  several  years  with  certain
modifications  incorporated to reflect changes in Pioneer USA's overall business
activities.

       Reclassifications - Certain  reclassifications  may have been made to the
1998 and 1997 financial  statements to conform to the 1999  financial  statement
presentations.

       Environmental  - The Partnership is subject to extensive  federal,  state
and local  environmental laws and regulations.  These laws, which are constantly
changing,  regulate  the  discharge of materials  into the  environment  and may
require the Partnership to remove or mitigate the  environmental  effects of the
disposal  or release of  petroleum  or  chemical  substances  at various  sites.
Environmental expenditures are expensed or capitalized depending on their future
economic benefit.  Expenditures  that relate to an existing  condition caused by
past  operations  and  that  have no  future  economic  benefits  are  expensed.
Liabilities  for   expenditures  of  a  noncapital   nature  are  recorded  when
environmental  assessment and/or  remediation is probable,  and the costs can be
reasonably  estimated.  Such liabilities are generally  undiscounted  unless the
timing of cash  payments for the  liability  or component  are fixed or reliably
determinable. No such liabilities have been accrued as of December 31, 1999.

       Revenue  recognition - The Partnership  uses the  entitlements  method of
accounting for crude oil and natural gas revenues.

Note 3.          Impairment of long-lived assets

       In accordance with SFAS 121, the  Partnership  reviews its proved oil and
gas properties  for  impairment  whenever  events and  circumstances  indicate a
decline in the recoverability of the carrying value of the Partnership's oil and
gas properties.  The Partnership has estimated the expected future cash flows of

                                       19


<PAGE>



its oil and gas  properties  as of December  31, 1999,  1998 and 1997,  based on
proved reserves, and compared such estimated future cash flows to the respective
carrying  amount of the oil and gas  properties  to  determine  if the  carrying
amounts were likely to be  recoverable.  For those proved oil and gas properties
for which the carrying  amount  exceeded  the  estimated  future cash flows,  an
impairment  was determined to exist;  therefore,  the  Partnership  adjusted the
carrying  amount  of  those  oil and gas  properties  to  their  fair  value  as
determined by discounting  their  expected  future cash flows at a discount rate
commensurate  with  the  risks  involved  in  the  industry.  As a  result,  the
Partnership  recognized non-cash impairment provisions of $66,898,  $306,043 and
$485,158  related to its proved oil and gas  properties  during  1999,  1998 and
1997, respectively.

Note 4.     Income taxes

       The  financial  statement  basis  of the  Partnership's  net  assets  and
liabilities was $1,088,512 greater than the tax basis at December 31, 1999.

       The following is a reconciliation  of net income (loss) per statements of
operations  with the net income per  Federal  income tax  returns  for the years
ended December 31:
                                                 1999        1998         1997
                                               --------   ---------   --------
Net income (loss) per statements of
  operations                                   $266,803   $(280,631)  $ (7,029)
Depletion and depreciation provisions for tax
  reporting purposes (greater than) less than
  amounts for financial reporting purposes      106,971     (11,376)   (56,924)
Impairment of oil and gas properties for
  financial reporting purposes                   66,898     306,043    485,158
Other, net                                       (4,139)      8,462     (3,418)
                                                -------    --------    -------
     Net income per Federal
       income tax returns                      $436,533   $  22,498   $417,787
                                                =======    ========    =======

Note 5.     Oil and gas producing activities

       The following is a summary of the costs incurred,  whether capitalized or
expensed,  related to the Partnership's oil and gas producing activities for the
years ended December 31:

                                            1999         1998          1997
                                          ---------    ---------    ---------

     Development costs                    $  23,360    $  15,294    $  14,946
                                           ========     ========     ========

       Capitalized oil and gas properties consist of the following:

                                                       1999           1998
                                                   -----------    -----------

     Property acquisition costs                    $   343,666    $   343,666
     Completed wells and equipment                   9,357,855      9,334,495
                                                    ----------     ----------
                                                     9,701,521      9,678,161
       Accumulated depletion                        (7,574,749)    (7,280,260)
                                                    ----------     ----------
                Net capitalized costs              $ 2,126,772    $ 2,397,901
                                                    ==========     ==========

                                       20


<PAGE>



Note 6.     Related party transactions

       Pursuant to the limited  partnership  agreement,  the Partnership had the
following  related party  transactions  with the managing general partner or its
affiliates during the years ended December 31:

                                                    1999       1998       1997
                                                  --------   --------   --------
 Payment of lease operating and supervision
   charges in accordance with standard industry
   operating agreements                           $213,969   $210,197   $244,169

 Reimbursement of general and administrative
   expenses                                       $ 34,110   $ 24,415   $ 42,044

       The Partnership  participates in oil and gas activities through an income
tax partnership (the "Program") pursuant to the Program agreement.  Pioneer USA,
P&P Employees 91-A GP ("EMPL") (the  "Entities") and the Partnership are parties
to the  Program  agreement.  EMPL is a  general  partnership  organized  for the
benefit of certain employees of Pioneer USA.

       The costs and  revenues of the Program are  allocated to the Entities and
the Partnership as follows:

                                                   Entities     Partnership (1)
                                                  -----------   ---------------
Revenues:
     Proceeds from disposition of depreciable
       and depletable properties                  34.3434343%      65.6565657%
     All other revenues                           34.3434343%      65.6565657%

Costs and expenses:
     Lease acquisition costs, drilling and
       completion costs and all other costs       24.2424242%      75.7575758%
     Operating costs, reporting and legal
       expenses and general and administrative
       expenses                                   34.3434343%      65.6565657%

     (1)  Includes Pioneer USA's 1% general partner ownership which is allocated
          at the  Partnership level and 45  limited partner  interests  owned by
          Pioneer USA.

Note 7.     Oil and gas information (unaudited)

       The following table presents  information  relating to the  Partnership's
estimated  proved oil and gas  reserves at  December  31, 1999 1998 and 1997 and
changes in such quantities  during the years then ended.  Due to a change in the
accounting policy of the managing general partner in 1997, the Partnership began
accounting  for processed  natural gas production in two  components:  processed
natural gas liquids ("NGLs") and dry residue gas. NGLs are reflected in "Oil and
NGLs" in the table below. All of the Partnership's reserves are proved developed
and located within the United States. The Partnership's reserves are based on an
evaluation prepared by Pioneer USA internal engineers and reviewed by Williamson
Petroleum Consultants, Inc., an independent petroleum consultant, using criteria
established by the Securities and Exchange Commission. Reserve value information
is available to limited  partners  pursuant to the  partnership  agreement  and,
therefore, is not presented.

                                       21


<PAGE>



                                                Oil and NGLs        Gas
                                                    (bbls)         (mcf)
                                                ------------     ----------

   Net proved reserves at January 1, 1997            770,418      2,258,818
   Revisions                                         200,381     (1,059,329)
   Production                                        (60,858)      (135,829)
                                                ------------     ----------
   Net proved reserves at December 31, 1997          909,941      1,063,660
   Revisions                                        (333,709)      (228,660)
   Production                                        (70,623)      (108,617)
                                                ------------     ----------
   Net proved reserves at December 31, 1998          505,609        726,383
   Revisions                                         430,559        692,465
   Production                                        (64,820)      (100,615)
                                                ------------     ----------
   Net proved reserves at December 31, 1999          871,348      1,318,233
                                                ============     ==========

       As of  December  31,  1999 the  estimated  present  value of  future  net
revenues of proved reserves, calculated using December 31, 1999 prices of $25.33
per  barrel  of oil,  $15.28  per  barrel  of  NGLs  and  $1.71  per mcf of gas,
discounted at 10% was approximately $4,938,000 and undiscounted was $9,599,000.

       Numerous  uncertainties exist in estimating quantities of proved reserves
and future net revenues therefrom.  The estimates of proved reserves and related
future net revenues  set forth in this report are based on various  assumptions,
which may ultimately  prove to be inaccurate.  Therefore,  such estimates should
not be construed as estimates of the current  market value of the  Partnership's
proved  reserves.   The  Partnership   emphasizes  that  reserve  estimates  are
inherently imprecise and,  accordingly,  the estimates are expected to change as
future information becomes available.

Note 8.     Major customers

       The following table reflects the major customers of the Partnership`s oil
and gas sales (a major  customer is defined as a customer whose sales exceed 10%
of total sales) during the years ended December 31:

                                             1999        1998        1997
                                           --------    --------    --------

            Plains All American Inc.          36%          -           -
            TEPPCO Crude Oil LLC              17%          -           -
            Genesis Crude Oil, L.P.           -           52%         55%
            Western Gas Resources, Inc.        5%         19%         18%

       At December 31,  1999,  the amounts  receivable  from Plains All American
Inc. and TEPPCO Crude Oil LLC were $52,962 and $21,252, respectively,  which are
included  in the  caption  "Accounts  receivable  - oil  and gas  sales"  in the
accompanying Balance Sheet.

       Pioneer USA is of the opinion  that the loss of any one  purchaser  would
not have an adverse effect on the ability of the Partnership to sell its oil and
gas production or natural gas products.

                                       22


<PAGE>



Note 9.     Organization and operations

       The Partnership was organized September 30, 1991 as a limited partnership
under the Delaware Act for the purpose of acquiring and  developing  oil and gas
properties.  The general partners were converted to limited partners on November
30, 1992. The managing  general partner  received an opinion of legal counsel to
the  effect  that such  conversion  will not  result  in  material  adverse  tax
consequences.  The  following  is  a  brief  summary  of  the  more  significant
provisions of the limited partnership agreement:

       Managing   general  partner  -  The  managing   general  partner  of  the
       Partnership  is Pioneer USA.  Pioneer USA has the power and  authority to
       manage,  control and  administer  all  Partnership  affairs.  As managing
       general  partner  and  operator  of  the  Partnership's  properties,  all
       production  expenses  are  incurred  by  Pioneer  USA and  billed  to the
       Partnership and a portion of revenue is initially received by Pioneer USA
       prior to being paid to the  Partnership.  Under the  limited  partnership
       agreement,  the managing  general  partner  pays 1% of the  Partnership's
       acquisition,  drilling and completion  costs, and 1% of its operating and
       general and administrative  expenses. In return it is allocated 1% of the
       Partnership's revenues.

       Limited  partner  liability  - The  maximum  amount of  liability  of any
       limited partner is the total contributions of such partner plus his share
       of any undistributed profits.

       Initial  capital  contributions  -  The  limited  partners  entered  into
       subscription   agreements   for  aggregate   capital   contributions   of
       $11,620,000.  The  managing  general  partner is required  to  contribute
       amounts equal to 1% of initial  Partnership capital less sales commission
       costs  allocated  to  the  limited  partners  and to  contribute  amounts
       necessary to pay costs and expenses allocated to it under the partnership
       agreement to the extent its share of revenues does not cover such costs.

ITEM 9.     Changes in and Disagreements with Accountants on Accounting
               and Financial Disclosure

None.

                                       23


<PAGE>



                                    PART III

ITEM 10.    Directors and Executive Officers of the Partnership

The  Partnership  does not have any  officers  or  directors.  Under the limited
partnership agreement,  the Partnership's managing general partner, Pioneer USA,
is  granted  the  exclusive  right and full  authority  to manage,  control  and
administer the Partnership's business.

Set forth below are the names, ages and positions of the directors and executive
officers of Pioneer USA. Directors of Pioneer USA are elected to serve until the
next annual meeting of  stockholders  or until their  successors are elected and
qualified.  During June 1999,  Mr. Lon C. Kile resigned as an officer of Pioneer
USA.  During  January  2000,  Mr. M. Garrett Smith also resigned his position as
Director and Chief Financial Officer of Pioneer USA. Mr. Timothy L. Dove assumed
the  responsibility  of Chief Financial Officer of Pioneer USA after Mr. Smith's
resignation.
                             Age at
                          December 31,
        Name                  1999                       Position
---------------------     ------------     -------------------------------

Scott D. Sheffield             47          President

Timothy L. Dove                43          Executive Vice President, Chief
                                             Financial Officer and Director

Dennis E. Fagerstone           50          Executive Vice President and Director

Mark L. Withrow                52          Executive Vice President, General
                                             Counsel and Director

Rich Dealy                     33          Vice President and Chief Accounting
                                             Officer

         Scott D. Sheffield.  Mr. Sheffield is a  graduate of  The University of
Texas with a B.S. in Petroleum Engineering.  Since August 1997, he has served as
President,  Chief  Executive  Officer and a director of Pioneer and President of
Pioneer  USA.  Mr.  Sheffield  assumed the  position of Chairman of the Board of
Pioneer in August 1999.  He served as a director of Pioneer USA from August 1997
until  his  resignation  from the  board in June  1999.  Mr.  Sheffield  was the
President and a director of Parker & Parsley from May 1990 until August 1997 and
was the  Chairman of the Board and Chief  Executive  Officer of Parker & Parsley
from  October  1990 until  August  1997.  He was the sole  director  of Parker &
Parsley from May 1990 until October 1990. Mr.  Sheffield joined Parker & Parsley
Development Company ("PPDC"),  a predecessor of Parker & Parsley, as a petroleum
engineer  in 1979.  He  served  as Vice  President  -  Engineering  of PPDC from
September 1981 until April 1985 when he was elected President and a director. In
March 1989, Mr.  Sheffield was elected Chairman of the Board and Chief Executive
Officer of PPDC. Before joining PPDC's  predecessor,  Mr. Sheffield was employed
as a production and reservoir engineer for Amoco Production Company.

                                       24


<PAGE>




       Timothy L. Dove.   Mr. Dove earned a B.S. in  Mechanical Engineering from
Massachusetts  Institute of Technology  in 1979 and received his M.B.A.  in 1981
from the  University of Chicago.  He became  Executive Vice President - Business
Development  of Pioneer and Pioneer USA in August 1997 and was also  appointed a
director of Pioneer USA in August  1997.  Mr. Dove assumed the position of Chief
Financial  Officer of Pioneer and Pioneer USA  effective  February 1, 2000.  Mr.
Dove joined Parker & Parsley in May 1994 as Vice President -  International  and
was promoted to Senior Vice President - Business Development in October 1996, in
which  position he served until August 1997.  Prior to joining Parker & Parsley,
Mr. Dove was employed with Diamond  Shamrock  Corp.,  and its  successor,  Maxus
Energy Corp, in various capacities in international  exploration and production,
marketing, refining and marketing and planning and development.

       Dennis E. Fagerstone.  Mr. Fagerstone,  a graduate of the Colorado School
of  Mines  with a B.S.  in  Petroleum  Engineering,  became  an  Executive  Vice
President  of Pioneer and Pioneer USA in August  1997.  He was also  appointed a
director of Pioneer USA in August 1997.  He served as Executive  Vice  President
and Chief  Operating  Officer of Mesa from March 1, 1997 until August 1997. From
October 1996 to February  1997, Mr.  Fagerstone  served as Senior Vice President
and Chief Operating Officer of Mesa and from May 1991 to October 1996, he served
as Vice President - Exploration  and  Production of Mesa.  From June 1988 to May
1991, Mr. Fagerstone served as Vice President - Operations of Mesa.

       Mark L. Withrow.  Mr. Withrow, a graduate of Abilene Christian University
with a B. S. in  Accounting  and Texas Tech  University  with a Juris  Doctorate
degree,  became  Executive  Vice  President,  General  Counsel and  Secretary of
Pioneer  and  Pioneer USA in August  1997.  He was also  appointed a director of
Pioneer USA in August 1997. Mr. Withrow was Vice President - General  Counsel of
Parker & Parsley from January 1991, when he joined Parker & Parsley,  to January
1995,  when he was appointed  Senior Vice  President - General  Counsel.  He was
Parker &  Parsley's  Secretary  from  August 1992 until  August  1997.  Prior to
joining Parker & Parsley,  Mr. Withrow was the managing  partner of the law firm
of Turpin, Smith, Dyer, Saxe & MacDonald, Midland, Texas.

       Rich Dealy. Mr. Dealy is a graduate of Eastern New Mexico University with
a B.B.A.  in Accounting  and Finance and is a Certified  Public  Accountant.  He
became Vice President and Chief Accounting Officer of Pioneer and Pioneer USA in
February 1998. Mr. Dealy served as Controller of Pioneer USA from August 1997 to
February  1998.  He served as Controller of Parker & Parsley from August 1995 to
August 1997. Mr. Dealy joined Parker & Parsley as an Accounting Manager in July,
1992. He was previously  employed with KPMG Peat Marwick as an Audit Senior,  in
charge of Parker & Parsley's audit.

ITEM 11.     Executive Compensation

The  Partnership  does not have any  directors  or officers.  Management  of the
Partnership  is  vested  in  the  managing  general  partner.   The  Partnership
participates in oil and gas activities  through an income tax  partnership  (the
"Program")  pursuant to the  Program  agreement.  Under the  Program  agreement,
Pioneer USA and P&P  Employees  91-A GP ("EMPL")  pay  approximately  25% of the
Program's  acquisition,  drilling and completion costs and  approximately 35% of
its  operating  and general and  administrative  expenses.  In return,  they are
allocated  approximately  35% of the  Program's  revenues.  See Notes 6 and 9 of
Notes to Financial Statements included in "Item 8.

                                       25


<PAGE>



Financial Statements and Supplementary Data" for information  regarding fees and
reimbursements  paid to the managing  general  partner or its  affiliates by the
Partnership.

Pioneer  USA's  current  executive  officers  and other  employees  are  general
partners of EMPL. EMPL serves as a co-general partner of the Program. Under this
arrangement, EMPL pays approximately 2.5% of the Program's acquisition, drilling
and  completion  costs and  approximately  3.5% of its operating and general and
administrative  expenses. In return, EMPL is allocated approximately 3.5% of the
Program's  revenues.  EMPL does not receive any fees or reimbursements  from the
Partnership.

The Partnership does not directly pay any salaries of the executive  officers or
employees of Pioneer USA,  but does pay a portion of Pioneer  USA's  general and
administrative  expenses of which these salaries are a part. See Note 6 of Notes
to  Financial   Statements  included  in  "Item  8.  Financial   Statements  and
Supplementary Data".

ITEM 12.     Security Ownership of Certain Beneficial Owners and Management

(a)    Beneficial owners of more than five percent

The Partnership is not aware of any person who  beneficially  owns 5% or more of
the outstanding  limited partnership  interests of the Partnership.  Pioneer USA
owned 45 limited partner interests at January 1, 2000.

(b)    Security ownership of management

The Partnership  does not have any officers or directors.  The managing  general
partner  of the  Partnership,  Pioneer  USA,  has the  exclusive  right and full
authority to manage,  control and administer the Partnership's  business.  Under
the limited  partnership  agreement,  limited partners holding a majority of the
outstanding  limited  partnership  interests  have  the  right  to take  certain
actions,  including  the removal of the  managing  general  partner or any other
general  partner.  The  Partnership  is not aware of any current  arrangement or
activity  which may lead to such removal.  The  Partnership  is not aware of any
officer or director of Pioneer USA who  beneficially  owns  limited  partnership
interests in the Partnership.

ITEM 13.     Certain Relationships and Related Transactions

Transactions with the managing general partner or its affiliates

Pursuant to the limited partnership agreement, the Partnership had the following
related party  transactions  with the managing general partner or its affiliates
during the years ended December 31:

                                                  1999        1998        1997
                                               ---------   ---------   ---------
 Payment of lease operating and supervision
    charges in accordance with standard
    industry operating agreements              $ 213,969   $ 210,197   $ 244,169

 Reimbursement of general and administrative
    expenses                                   $  34,110   $  24,415   $  42,044

                                       26


<PAGE>




Under the limited partnership agreement, the managing general partner pays 1% of
the  Partnership's  acquisition,  drilling  and  completion  costs and 1% of its
operating and general and administrative expenses. In return, it is allocated 1%
of the  Partnership's  revenues.  Also,  see Notes 6 and 9 of Notes to Financial
Statements  included in "Item 8. Financial  Statements and Supplementary  Data",
regarding  the  Partnership's  participation  in oil and gas  activities  of the
Program.

                                       27


<PAGE>



                                     PART IV

ITEM 14.     Exhibits, Financial Statement Schedules and Reports on Form 8-K.

(a)    1.    Financial statements

             The following are filed as part of this annual report:

                Independent Auditors' Report - Ernst & Young LLP

                Independent Auditors' Report - KPMG LLP

                Balance sheets as of December 31, 1999 and 1998

                Statements of operations for the years ended December 31, 1999,
                  1998 and 1997

                Statements of partners' capital for the years ended December 31,
                  1999, 1998 and 1997

                Statements of cash flows for the years ended December 31, 1999,
                  1998 and 1997

                Notes to financial statements

       2.    Financial statement schedules

             All  financial  statement  schedules  have  been omitted  since the
             required  information  is in  the  financial  statements  or  notes
             thereto, or is not applicable nor required.

(b)    Reports on Form 8-K

None.

(c)    Exhibits

       The exhibits  listed on the  accompanying  index to exhibits are filed or
       incorporated by reference as part of this annual report.

                                       28


<PAGE>



                               S I G N A T U R E S


       Pursuant to the requirements of the Securities  Exchange Act of 1934, the
Registrant  has duly  caused  this  report  to be  signed  on its  behalf by the
undersigned, thereunto duly authorized.

                                          PARKER & PARSLEY 91-A, L.P.

Dated:  March 23, 2000            By:     Pioneer Natural Resources USA, Inc.
                                            Managing General Partner


                                          By:     /s/ Scott D. Sheffield
                                                  ----------------------------
                                                  Scott D. Sheffield, President

       Pursuant to the requirements of the Securities Exchange Act of 1934, this
report  has  been  signed  below  by the  following  persons  on  behalf  of the
Registrant and in the capacities and on the date indicated.


/s/ Scott D. Sheffield      President of Pioneer USA              March 23, 2000
------------------------
Scott D. Sheffield


/s/ Timothy L. Dove         Executive Vice President, Chief       March 23, 2000
------------------------    Financial Officer and Director of
Timothy L. Dove             Pioneer USA


/s/ Dennis E. Fagerstone    Executive Vice President and          March 23, 2000
------------------------    Director of Pioneer USA
Dennis E. Fagerstone


/s/ Mark L. Withrow         Executive Vice President, General     March 23, 2000
------------------------    Counsel and Director of Pioneer USA
Mark L. Withrow


/s/ Rich Dealy              Vice President and Chief Accounting   March 23, 2000
------------------------    Officer of Pioneer USA
Rich Dealy

                                       29


<PAGE>


                           PARKER & PARSLEY 91-A, L.P.

                                INDEX TO EXHIBITS


       The following documents are incorporated by reference in response to Item
14(c):

Exhibit No.                       Description                           Page

    3(a)         Form of Agreement of Limited Partnership                -
                 of Parker & Parsley 91-A, L.P. incorporated
                 by reference to Exhibit A of the Partnership's
                 Registration Statement on Form S-1
                 (Registration No. 33-38582) (hereinafter
                 called the Partnership's Registration Statement)

    4(b)         Form of Limited Partner Subscription Agree-             -
                 ment incorporated by reference to Exhibit C of
                 the Partnership's Registration Statement

    4(b)         Form of General Partner Subscription Agree-             -
                 ment incorporated by reference to Exhibit D of
                 the Partnership's Registration Statement

    4(b)         Power of Attorney incorporated by reference to          -
                 Exhibit B of the Partnership's Registration
                 Statement

    4(c)         Specimen Certificate of Limited Partnership             -
                 Interest incorporated by reference to Exhibit 4.3
                 of Amendment No. 2 of the Partnership's
                 Registration Statement

   10(b)         Development Drilling Program Agreement                  -
                 incorporated by reference to Exhibit B of the
                 Partnership's Registration Statement

   27.1*         Financial Data Schedule

*Filed herewith

                                       30


<PAGE>