<PAGE>

                      SECURITIES AND EXCHANGE COMMISSION
                      ----------------------------------
                            Washington, D.C. 20549
                            ----------------------

                                  FORM 10-Q/A
(Mark One)
----------

[X]  QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE
     ACT OF 1934 FOR THE QUARTERLY PERIOD ENDED MARCH 31, 2001 OR

[ ]  TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
     EXCHANGE ACT OF 1934 FOR THE TRANSITION PERIOD FROM _________________ TO
     _________________


                        Commission file number 1-10389
                        ------------------------------

                          WESTERN GAS RESOURCES, INC.
                          ---------------------------
            (Exact name of registrant as specified in its charter)


            Delaware                                          84-1127613
-------------------------------------       ------------------------------------
(State or other jurisdiction of             (I.R.S. Employer Identification No.)
 incorporation or organization)

12200 N. Pecos Street, Denver, Colorado                      80234-3439
--------------------------------------------------------------------------------
(Address of principal executive offices)                     (Zip Code)

                                (303) 452-5603
--------------------------------------------------------------------------------
              Registrant's telephone number, including area code

                                  No changes
--------------------------------------------------------------------------------
       (Former name, former address and former fiscal year, if changed
                              since last report).

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
                                      ---
On May 1, 2001, there were 32,497,723 shares of the registrant's Common Stock
outstanding.

This Form 10-Q/A for the quarter ending March 31, 2001 is being filed to correct
the Consolidated Balance Sheet line item for Treasury Stock, at cost, located in
Part I, Item 1.

                                       1
<PAGE>

                          Western Gas Resources, Inc.
                                   Form 10-Q
                               Table of Contents

<TABLE>
<CAPTION>
PART I - Financial Information                                                                                        Page
------------------------------                                                                                        ----

     Item 1.    Financial Statements
     <S>        <C>                                                                                                     <C>
                Consolidated Balance Sheet - March 31, 2001 and December 31, 2000....................................    3

                Consolidated Statement of Cash Flows - Three Months Ended March 31, 2001
                and 2000.............................................................................................    4

                Consolidated Statement of Operations - Three Months Ended March 31, 2001 and 2000....................    5

                Consolidated Statement of Changes in Stockholders' Equity - Three Months Ended
                March 31, 2001.......................................................................................    6

                Notes to Consolidated Financial Statements...........................................................    7


     Signatures......................................................................................................   11
</TABLE>

                                       2
<PAGE>

PART I - FINANCIAL INFORMATION

Item 1.     Financial Statements
            --------------------

                          WESTERN GAS RESOURCES, INC.
                          CONSOLIDATED BALANCE SHEET
                   (Dollars in thousands, except share data)

<TABLE>
<CAPTION>
                                                                                               March 31,      December 31,
       ASSETS                                                                                   2001               2000
       ------                                                                               ------------      ------------
                                                                                             (unaudited)
<S>                                                                                         <C>              <C>
Current assets:
    Cash and cash equivalents.........................................................      $     74,387      $     12,927
    Trade accounts receivable, net....................................................           356,393           546,791
    Product inventory.................................................................            14,364            44,822
    Parts inventory...................................................................             3,102             3,489
    Assets from price risk management activities......................................            31,156                 -
    Assets held for sale..............................................................                 -            25,001
    Other ............................................................................             2,953             2,654
                                                                                            ------------      ------------
       Total current assets...........................................................           482,355           635,684
                                                                                            ------------      ------------
Property and equipment:
    Gas gathering, processing, storage and transportation.............................           862,690           856,982
    Oil and gas properties and equipment (successful efforts method)..................           156,864           139,084
    Construction in progress..........................................................            57,457            58,319
                                                                                            ------------      ------------
                                                                                               1,077,011         1,054,385
    Less:  Accumulated depreciation, depletion and amortization.......................          (320,096)         (306,651)
                                                                                            ------------      ------------
       Total property and equipment, net..............................................           756,915           747,734
                                                                                            ------------      ------------
Other assets:
    Gas purchase contracts (net of accumulated amortization of $33,840 and
       $33,357, respectively).........................................................            34,316            34,798
    Assets from price risk management activities......................................               670                 -
    Other ............................................................................            14,327            13,206
                                                                                            ------------      ------------
    Total other assets................................................................            49,313            48,004
                                                                                            ------------      ------------
TOTAL ASSETS..........................................................................      $  1,288,583      $  1,431,422
                                                                                            ============      ============
       LIABILITIES AND STOCKHOLDERS' EQUITY
       ------------------------------------
Current liabilities:
    Accounts payable..................................................................      $    401,309      $    581,563
    Accrued expenses..................................................................            35,276            25,094
    Liabilities from price risk management activities.................................            39,728                 -
    Dividends payable.................................................................             4,209             4,205
                                                                                            ------------      ------------
       Total current liabilities......................................................           480,522           610,862
Long-term debt........................................................................           305,000           358,700
Liabilities from price risk management activities.....................................               385                 -
Other long-term liabilities...........................................................             2,559             2,646
Deferred income taxes payable, net....................................................            79,550            67,680
                                                                                            ------------      ------------
Total liabilities.....................................................................           868,016         1,039,888
                                                                                            ------------      ------------
Stockholders' equity:
    Preferred Stock; 10,000,000 shares authorized:
       $2.28 cumulative preferred stock, par value $.10; 1,400,000 shares issued
          ($35,000,000 aggregate liquidation preference)..............................               140               140
       $2.625 cumulative convertible preferred stock, par value $.10; 2,760,000
          issued ($138,000,000 aggregate liquidation preference)......................               276               276
    Common stock, par value $.10; 100,000,000 shares authorized; 32,497,723 and
          32,361,131 shares issued, respectively .....................................             3,275             3,265
    Treasury stock, at cost; 25,016 common shares and 44,290 shares of $2.28 cumulative
          preferred stock in treasury.................................................            (1,907)           (1,778)
    Additional paid-in capital........................................................           401,102           400,157
    Retained earnings (deficit).......................................................            24,561           (11,820)
    Accumulated other comprehensive income............................................            (5,996)            2,178
    Notes receivable from key employees secured by common stock.......................              (884)             (884)
                                                                                            ------------      ------------
       Total stockholders' equity.....................................................           420,567           391,534
                                                                                            ------------      ------------
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY............................................      $  1,288,583      $  1,431,422
                                                                                            ============      ============
</TABLE>

       The accompanying notes are an integral part of the consolidated
                             financial statements.

                                       3
<PAGE>

                          WESTERN GAS RESOURCES, INC.
                     CONSOLIDATED STATEMENT OF CASH FLOWS
                                  (Unaudited)
                            (Dollars in thousands)
<TABLE>
<CAPTION>
                                                                                                  Three Months Ended
                                                                                                       March 31,
                                                                                            ------------------------------
                                                                                                2001              2000
                                                                                            ------------      ------------
<S>                                                                                         <C>              <C>
Reconciliation of net income to net cash provided by operating activities:
Net income............................................................................      $     40,590      $     13,006
Add income items that do not affect cash:
    Depreciation, depletion and amortization..........................................            14,478            13,309
    Gain on the sale of property and equipment........................................           (11,223)           (5,299)
    Deferred income taxes.............................................................            16,719             7,441
    Non-cash change in fair value of derivatives......................................            (5,049)                -
    Other non-cash items, net.........................................................                 8               929
                                                                                            ------------      ------------
                                                                                                  55,523            29,386
                                                                                            ------------      ------------

Adjustments to working capital to arrive at net cash provided by operating
    activities:
    (Increase) decrease in trade accounts receivable..................................           190,454           (31,234)
    Decrease in product inventory ....................................................            30,458            22,762
    Decrease in parts inventory ......................................................               387             1,066
    (Increase) decrease in other current assets.......................................              (299)            7,937
    Decrease in other assets and liabilities, net.....................................             8,076                37
    Decrease in accounts payable......................................................          (180,254)           (5,614)
    (Increase) decrease in accrued expenses...........................................             2,199           (14,975)
                                                                                            ------------      ------------

Net cash provided by operating activities.............................................           106,544             9,365
                                                                                            ------------      ------------
Cash flows from investing activities:
    Purchases of property and equipment...............................................           (25,908)          (28,498)
    Proceeds from the dispositions of property and equipment .........................            38,075            15,057
    Contributions to equity investees.................................................              (169)                -
                                                                                            ------------      ------------

Net cash provided by (used in) investing activities...................................            11,998           (13,441)
                                                                                            ------------      ------------

Cash flows from financing activities:
    Proceeds from exercise of common stock options....................................               955                71
    Repurchase of $2.28 cumulative preferred stock....................................              (129)                -
    Payments on revolving credit facility.............................................          (301,953)         (293,286)
    Borrowings under revolving credit facility........................................           248,250           291,350
    Payments on notes.................................................................                 -                 -
    Dividends paid....................................................................            (4,205)           (4,217)
                                                                                            ------------      ------------

Net cash used in financing activities.................................................           (57,082)           (6,082)
                                                                                            ------------      ------------
Net increase (decrease) in cash and cash equivalents..................................            61,460           (10,158)
Cash and cash equivalents at beginning of period......................................            12,927            14,062
                                                                                            ------------      ------------
Cash and cash equivalents at end of period ...........................................      $     74,387      $      3,904
                                                                                            ============      ============
</TABLE>



       The accompanying notes are an integral part of the consolidated
                             financial statements.

                                       4
<PAGE>

                          WESTERN GAS RESOURCES, INC.
                     CONSOLIDATED STATEMENT OF OPERATIONS
                                  (Unaudited)
          (Dollars in thousands, except share and per share amounts)
<TABLE>
<CAPTION>


                                                                                                  Three Months Ended
                                                                                                       March 31,
                                                                                            ------------------------------
                                                                                                2001              2000
                                                                                            -------------    -------------
<S>                                                                                        <C>              <C>
Revenues:
    Sale of gas....................................................................         $   1,044,876    $     413,816
    Sale of natural gas liquids....................................................               129,477          135,388
    Processing, transportation and storage revenue.................................                16,036           13,885
    Unrealized gain on marketing activities........................................                 5,049                -
    Other, net.....................................................................                 1,838            2,063
                                                                                            -------------    -------------

       Total revenues..............................................................             1,197,276          565,152
                                                                                            -------------    -------------

Costs and expenses:
    Product purchases..............................................................             1,086,600          501,143
    Plant operating expense........................................................                17,037           15,262
    Oil and gas exploration and production expense.................................                 9,605            4,146
    Depreciation, depletion and amortization ......................................                14,478           13,309
    Gain on sale of assets ........................................................               (11,223)          (5,299)
    Selling and administrative expense.............................................                 8,479            7,389
    Interest expense...............................................................                 6,829            8,218
                                                                                            -------------    -------------

       Total costs and expenses....................................................             1,131,805          544,168
                                                                                            -------------    -------------

Income before taxes................................................................                65,471           20,984

Provision for income taxes:
    Current .......................................................................                 8,162              537
    Deferred.......................................................................                16,719            7,441
                                                                                            -------------    -------------

          Total provision for income taxes.........................................                24,881            7,978
                                                                                            -------------    -------------

Net income.........................................................................                40,590           13,006

Preferred stock requirements.......................................................                (2,584)          (2,610)
                                                                                            -------------    -------------

Income attributable to common stock................................................         $      38,006    $      10,396
                                                                                            =============    =============

Earnings per share of common stock.................................................         $        1.17    $         .32
                                                                                            =============    =============

Weighted average shares of common stock outstanding................................            32,405,044       32,165,868
                                                                                            =============    =============

Income attributable to common stock  - assuming dilution...........................         $      39,817    $      10,396
                                                                                            =============    =============

Earnings per share of common stock-assuming dilution...............................         $        1.08    $         .32
                                                                                            =============    =============

Weighted average shares of common stock outstanding-assuming dilution..............            36,757,118       32,459,209
                                                                                            =============    =============
</TABLE>

       The accompanying notes are an integral part of the consolidated
                             financial statements.

                                       5
<PAGE>

                          WESTERN GAS RESOURCES, INC.
           CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY
                 (Dollars in thousands, except share amounts)

<TABLE>
<CAPTION>
                                                          Shares of       Shares of       $2.625
                                                           $2.28           $2.28        Cumulative                   Shares
                                                         Cumulative      Cumulative     Convertible    Shares       of Common
                                                         Preferred    Preferred Stock    Preferred   of Common        Stock
                                                           Stock        in Treasury        Stock        Stock      in Treasury
                                                         ----------   ---------------   -----------  ----------    -----------
<S>                                                      <C>          <C>               <C>          <C>           <C>
Balance at December 31, 2000 .........................    1,400,000           39,190    2,760,000    32,361,131         25,016
Comprehensive income:
   Net income, three months ended
     March 31, 2001 ..................................           --               --           --            --             --
   Translation adjustments ...........................           --               --           --            --             --
   Cumulative effect of change in accounting
     principle - January 1, 2001 .....................           --               --           --            --             --
   Reclassification adjustment for settled
     contracts .......................................           --               --           --            --             --
   Changes in fair value of outstanding hedging
     positions .......................................           --               --           --            --             --

   Total comprehensive income, net of tax ............

Stock options exercised ..............................           --               --           --       136,592             --
Tax benefit related to stock options .................           --               --           --            --             --
Loans forgiven .......................................           --               --           --            --             --
Dividends declared on common stock ...................           --               --           --            --             --
Dividends declared on $2.28 cumulative
   preferred stock ...................................           --               --           --            --             --
Dividends declared on $2.625 cumulative
   convertible preferred stock .......................           --               --           --            --             --
Repurchase of $2.28 cumulative
   preferred stock ...................................           --            5,100           --            --             --
                                                         ----------   --------------   ----------    ----------   ------------

Balance at March 31, 2001 ............................    1,400,000           44,290    2,760,000    32,497,723         25,016
                                                         ==========   ==============   ==========    ==========   ============


<CAPTION>
                                                                             $2.625
                                                                $2.28     Cumulative
                                                             Cumulative    Convertible                               Additional
                                                             Preferred     Preferred        Common      Treasury      Paid-In
                                                               Stock        Stock            Stock        Stock       Capital
                                                            -----------   ------------      ------      ---------   ------------
<S>                                                         <C>           <C>               <C>         <C>         <C>
Balance at December 31, 2000 .........................              140           276         3,265       (1,778)      400,157
Comprehensive income:
   Net income, three months ended
     March 31, 2001 ..................................               --            --            --           --            --
   Translation adjustments ...........................               --            --            --           --            --
   Cumulative effect of change in accounting
     principle - January 1, 2001 .....................               --            --            --           --            --
   Reclassification adjustment for settled
     contracts .......................................               --            --            --           --            --
   Changes in fair value of outstanding hedging
     positions .......................................               --            --            --           --            --


   Total comprehensive income, net of tax ............


Stock options exercised ..............................               --            --            10           --           945
Tax benefit related to stock options .................               --            --            --           --            --
Loans forgiven .......................................               --            --            --           --            --
Dividends declared on common stock ...................               --            --            --           --            --
Dividends declared on $2.28 cumulative
   preferred stock ...................................               --            --            --           --            --
Dividends declared on $2.625 cumulative
   convertible preferred stock .......................               --            --            --           --            --
Repurchase of $2.28 cumulative

   preferred stock ...................................               --            --            --         (129)           --
                                                                   ----     ---------     ---------   ----------    ----------

Balance at March 31, 2001 ............................             $140     $     276     $   3,275   $   (1,907)   $  401,102
                                                                   ====     =========     =========   ==========    ==========

<CAPTION>
                                                                          Accumulated
                                                                            Other         Notes         Total
                                                           Retained      Comprehensive  Receivable      Stock-
                                                          (Deficit)        Income       from Key      holders'
                                                          Earnings       Net of Tax     Employees      Equity
                                                          ---------      -------------  ----------    --------
<S>                                                       <C>            <C>            <C>           <C>
Balance at December 31, 2000 .........................     (11,820)        2,178          (884)        391,534
Comprehensive income:
   Net income, three months ended
     March 31, 2001 ..................................      40,590            --            --          40,590
   Translation adjustments ...........................          --           256            --             256
   Cumulative effect of change in accounting
     principle - January 1, 2001 .....................          --       (22,527)           --         (22,527)
   Reclassification adjustment for settled
     contracts .......................................          --        13,429            --          13,429
   Changes in fair value of outstanding hedging
     positions .......................................          --           668            --             668

                                                                                                    ----------

   Total comprehensive income, net of tax ............                                                  32,416
                                                                                                    ----------

Stock options exercised ..............................          --            --            --             955
Tax benefit related to stock options .................          --            --            --              --
Loans forgiven .......................................          --            --            --              --
Dividends declared on common stock ...................      (1,625)           --            --          (1,625)
Dividends declared on $2.28 cumulative
   preferred stock ...................................        (773)           --            --            (773)
Dividends declared on $2.625 cumulative
   convertible preferred stock .......................      (1,811)           --            --          (1,811)
Repurchase of $2.28 cumulative

   preferred stock ...................................          --            --            --            (129)
                                                        ----------    ----------    ----------      ----------

Balance at March 31, 2001 ............................  $   24,561    $   (5,996)   $     (884)     $  420,567
                                                        ==========    ==========    ==========      ==========
</TABLE>

       The accompanying notes are an integral part of the consolidated
                             financial statements.

                                       6
<PAGE>

                          WESTERN GAS RESOURCES, INC.
                  NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                                  (Unaudited)

GENERAL

    The interim consolidated financial statements presented herein should be
read in conjunction with the Consolidated Financial Statements and Notes thereto
included in our Annual Report on Form 10-K for the year ended December 31, 2000.
The interim consolidated financial statements as of March 31, 2001 and for the
three month periods ended March 31, 2001 and 2000 included herein are unaudited
but reflect, in the opinion of management, all adjustments (which include only
normal recurring adjustments) necessary to fairly present the results for such
periods. The results of operations for the three months ended March 31, 2001 are
not necessarily indicative of the results of operations expected for the year
ended December 31, 2001.

    Prior period amounts in the interim consolidated financial statements and
notes have been reclassified as appropriate to conform to the presentation used
in 2001.

EARNINGS PER SHARE OF COMMON STOCK

Earnings per share of common stock is computed by dividing income attributable
to common stock by the weighted average shares of common stock outstanding. In
addition, earnings per share of common stock - assuming dilution is computed by
dividing income attributable to common stock by the weighted average shares of
common stock outstanding as adjusted for potential common shares. Income
attributable to common stock is income less preferred stock dividends. We
declared preferred stock dividends of $2.6 million for each of the three-month
periods ended March 31, 2001 and 2000, respectively. Common stock options and
our $2.625 Cumulative Convertible Preferred Stock, which are potential common
shares, had a dilutive effect on earnings and increased the weighted average
number of shares of common stock outstanding by 4,352,074 and 293,341 for the
three-month periods ended March 31, 2001and 2000, respectively. The numerators
and the denominators for these periods were adjusted to reflect these potential
shares in calculating fully diluted earnings per share.

OTHER INFORMATION

    Bethel Treating Facility. In December 2000, we signed an agreement with
Anadarko Petroleum Corporation for the sale of all the outstanding stock of our
wholly-owned subsidiary, Pinnacle Gas Treating, Inc. ("Pinnacle") for $38.0
million. The only asset of this subsidiary was a 300 MMcf per day treating
facility and 86 miles of associated gathering assets located in east Texas. The
sale closed in January 2001 and resulted in a net pre-tax gain for financial
reporting purposes of $11.2 million in the first quarter of 2001.

    Western Gas Resources-California, Inc. In January 2000, we sold all the
outstanding stock of our wholly-owned subsidiary, Western Gas
Resources-California, Inc. ("WGR-California") for $14.9 million. The only asset
of this subsidiary was a 162-mile pipeline in the Sacramento basin of
California. We acquired the pipeline through the exercise of an option in a
transaction, which closed simultaneously with the sale of WGR-California. We
recognized a pre-tax gain on the sale of approximately $5.3 million in the first
quarter of 2000.

    The proceeds from these sales were initially used to reduce borrowings
outstanding on the Revolving Credit Facility.

    Westana. In February 2000, we acquired the remaining 50% interest in the
Westana Gathering Company for a net purchase price of $9.8 million. The results
from our ownership through February 2000 of a 50% equity interest in the Westana
Gathering Company are reflected in revenues in Other, net on the Consolidated
Statement of Operations. Beginning in March 2000, the results of these
operations are fully consolidated and are included in Revenues and Costs and
expenses. Additionally, in March 2000, our investment in the Westana Gathering
Company was reclassed from Other assets to Property and equipment.

DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES

    In June 1998, the Financial Accounting Standards Board, the FASB, issued
SFAS No. 133, "Accounting for Derivative Instruments and Hedging Activities"
("SFAS No. 133"), effective for fiscal years beginning after June 15, 2000.
Under SFAS No. 133, which was subsequently amended by SFAS No. 138, we are
required to recognize the change in the market value of all derivatives as
either assets or liabilities in the statement of financial position and measure
those instruments at fair value. Changes in the fair value of derivatives are
recorded each period in current earnings or other comprehensive income depending
upon the nature of the underlying transaction. Also on January 1, 2001, we
adopted mark-to-market accounting for the

                                       7
<PAGE>

remainder of our marketing activities which, for various reasons, are not
designated or qualified as hedges under SFAS 133. Upon the adoption of SFAS No.
133 and mark-to-market accounting on January 1, 2001, the impact was a decrease
in a component of stockholders' equity through Accumulated other comprehensive
income of $22.5 million, an increase to Current assets of $52.6 million, an
increase to Current liabilities of $86.9 million, an increase to Other long-term
liabilities of $1.1 million and a decrease in Deferred income taxes payable of
$12.9 million.

     Of the $22.5 million decrease to Accumulated other comprehensive income
resulting from the January 1, 2001 adoption of SFAS 133, $14.1 million was
reversed in the first quarter of 2001 with gains and losses from the underlying
transactions recognized through Total revenues. An additional $7.1 million of
this transition entry is currently anticipated to be recognized through Total
revenues in the remaining three quarters of 2001.

    The non-cash impact to our results of operations in the first quarter of
2001 resulting from the adoption of mark-to-market accounting for our marketing
activities resulted in additional pre-tax income of $5.0 million.

ADOPTION OF STOCKHOLDER RIGHTS PLAN

    In the first quarter of 2001, we adopted a Stockholder Rights Plan under
which rights were distributed as a dividend at the rate of one right for each
share of our common stock held by stockholders of record as of the close of
business on April 9, 2001. The Rights Plan was not adopted in response to any
efforts to acquire control of our company. The Rights Plan, however, is designed
to deter coercive takeover tactics including the accumulation of shares in the
open market or through private transactions and to prevent an acquirer from
gaining control of our company without offering a fair and adequate price to all
of our stockholders.

    Each right initially will entitle stockholders to buy one unit consisting of
1/100/th/ of a share of a share of a new series of preferred stock for $180 per
unit. The right generally will be exercisable only if a person or group acquires
beneficial ownership of 15 percent or more of our then outstanding common stock
or commences a tender or exchange offer upon consummation of which a person or
group would beneficially own 15 percent or more of our then outstanding common
stock. The rights will expire on March 22, 2011.

SUPPLEMENTARY CASH FLOW INFORMATION

    Interest paid was $4.2 million and $4.7 million for the three months ended
March 31, 2001 and 2000, respectively.

    No income taxes were paid during the three months ended March 31, 2001 or
the three months ended March 31, 2000.

SEGMENT REPORTING

    We operate in four principal business segments, as follows: Gas Gathering
and Processing, Production, Marketing and Transmission. These segments are
separately monitored by management for performance against our internal forecast
and are consistent with our internal financial reporting package. These segments
have been identified based upon the differing products and services, regulatory
environment and the expertise required for these operations.

    In our Gas Gathering and Processing segment, we connect producers' wells to
our gathering systems for delivery to our processing or treating plants, process
the natural gas to extract NGLs and treat the natural gas in order to meet
pipeline specifications. Our Marketing segment sells the residue gas and NGLs
extracted at our processing facilities.

    The activities of our Production segment include the exploration and
development of gas properties primarily in basins where our facilities are
located. The Marketing segment sells the majority of the production from these
properties.

    Our Marketing segment buys and sells gas and NGLs nationwide and in Canada
from or to a variety of customers. In addition, this segment also markets gas
and NGLs produced by our gathering, processing and production assets. Our
Canadian marketing operations, which are immaterial for separate presentation,
are included in this segment. The Marketing segment also includes losses
associated with our equity gas and NGL hedging program of $(14.5) million and
$(3.1) million for the quarters ended March 31, 2001 and March 31, 2000,
respectively.

    The Transmission segment reflects the operations of the MIGC and MGTC
pipelines. The majority of the revenue presented in this segment is derived from
transportation of residue gas.

                                       8
<PAGE>

     The following table sets forth our segment information as of and for the
quarters ended March 31, 2001 and 2000 (dollars in thousands). Due to our
integrated operations, the use of allocations in the determination of business
segment information is necessary. Inter-segment revenues are valued at prices
comparable to those of unaffiliated customers.

<TABLE>
<CAPTION>
                                         Gas
                                      Gathering                                                        Elim-
                                         and                                  Trans-                  inating
                                      Processing   Production    Marketing    mission  Corporate      Entries        Total
                                      -----------  ----------   ----------    -------  ---------      -------        ------
<S>                                   <C>          <C>        <C>          <C>         <C>         <C>         <C>
Quarter ended March 31, 2001
Revenues from unaffiliated
customers...........................  $    14,035  $     721  $ 1,209,769  $    2,675  $    282    $        -  $ 1,227,482
Interest income.....................            -          -            -           -     5,086        (4,802)         284
Other, net..........................            4         (1)     (31,872)          2     1,377             -      (30,490)
Intersegment sales..................      318,073     52,150       11,486       4,267        14      (385,990)           -
                                      -----------  ---------  -----------  ----------  --------    ----------  -----------
Total revenues......................      332,112     52,870    1,189,383       6,944     6,759      (390,792)   1,197,276
                                      -----------  ---------  -----------  ----------  --------    ----------  -----------
Product purchases...................      272,839      2,645    1,188,165        (434)       13      (376,628)   1,086,600
Plant operating expense.............       15,320         30           39       2,094       375          (821)      17,037
Oil and gas exploration
    and production expense..........            -     17,054            -           -         -        (7,449)       9,605
                                      -----------  ---------  -----------  ----------  --------    ----------  -----------
Operating profit....................  $    43,953  $  33,141  $     1,179  $    5,284  $  6,371    $   (5,894) $    84,034
                                      ===========  =========  ===========  ==========  ========    ==========  ===========

Depreciation, depletion and
amortization........................        9,500      3,067           40         415     1,456             -       14,478
Interest expense....................                                                                                 6,829
Gain on sale of assets..............                                                                               (11,223)
Selling and administrative expense..                                                                                 8,479
                                                                                                               -----------
Income before income taxes..........                                                                           $    65,471
                                                                                                               ===========

Identifiable assets.................  $   563,850  $ 141,430  $        57  $   46,808  $ 55,384    $        -  $   807,529
                                      ===========  =========  ===========  ==========  ========    ==========  ===========
</TABLE>

<TABLE>
<CAPTION>
                                                Gas
                                            Gathering                                                        Elim-
                                               and                                  Trans-                  inating
                                            Processing   Production    Marketing    mission  Corporate      Entries        Total
                                            -----------  ----------    ---------    -------  ---------      -------        ------
<S>                                         <C>          <C>        <C>          <C>         <C>         <C>         <C>
Quarter ended March 31, 2000
Revenues from unaffiliated
customers...........................         $   11,406   $    986  $   551,322  $    2,378  $     26    $       (8) $   566,110
Interest income.....................                 33          2           24           -     5,838        (5,668)         229
Other, net..........................                (20)         -       (2,203)          -     1,036             -       (1,187)
Intersegment sales..................            134,342     10,304       26,333       4,404         4      (175,387)           -
                                            -----------  ---------  -----------  ----------  --------    ----------  -----------
Total revenues......................            145,761     11,292      575,476       6,782     6,904      (181,063)     565,152
                                            -----------  ---------  -----------  ----------  --------    ----------  -----------
Product purchases...................            103,008        504      572,687           -       (25)     (175,031)     501,143
Plant operating expense.............             13,062         17            -       2,268       256          (341)      15,262
Oil and gas exploration
    and production expense..........                  -      4,146            -           -         -             -        4,146
                                            -----------  ---------  -----------  ----------  --------    ----------  -----------
Operating profit....................        $    29,691  $   6,625  $     2,789  $    4,514  $  6,673    $   (5,691) $    44,601
                                            ===========  =========  ===========  ==========  ========    ==========  ===========

Depreciation, depletion and
amortization........................              8,571      2,889           40         424     1,385             -       13,309
Interest expense....................                                                                                       8,218
Gain on sale of assets..............                                                                                      (5,299)
Selling and administrative expense..                                                                                       7,389
                                                                                                                     -----------
Income before income taxes..........                                                                                 $    20,984
                                                                                                                     ===========

Identifiable assets.................        $   547,611  $ 101,802  $        75  $   47,213  $ 37,710    $        -  $   734,411
                                            ===========  =========  ===========  ==========  ========    ==========  ===========
</TABLE>

                                       9
<PAGE>

LEGAL PROCEEDINGS

    Reference is made to "Part II - Other Information - Item 1. Legal
Proceedings," of this Form 10-Q.

                                       10
<PAGE>

SIGNATURES

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.

                                        WESTERN GAS RESOURCES, INC.
                                        ---------------------------
                                        (Registrant)


Date: May 23, 2001                      By:/s/ LANNY F. OUTLAW
                                           -------------------------------
                                           Lanny F. Outlaw
                                           Chief Executive Officer and President

Date: May 23, 2001                      By:/s/WILLIAM J. KRYSIAK
                                           -------------------------------
                                           William J. Krysiak
                                           Vice President - Finance
                                           (Principal Financial and Accounting
                                           Officer)

                                       11