<PAGE>   1

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                                 UNITED STATES

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                   FORM 10-Q

(MARK ONE)

[X]   QUARTERLY REPORT UNDER SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT
      OF 1934

FOR THE QUARTERLY PERIOD ENDED MARCH 31, 2000.

                                       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 NO. 2-80930

                        DEL TACO RESTAURANT PROPERTIES I
                        A CALIFORNIA LIMITED PARTNERSHIP
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

<TABLE>
<S>                                                     <C>
                      CALIFORNIA                                  95-3852699
            (STATE OR OTHER JURISDICTION OF                    (I.R.S. EMPLOYER
            INCORPORATION OR ORGANIZATION)                  IDENTIFICATION NUMBER)

 23041 AVENIDA DE LA CARLOTA, LAGUNA HILLS, CALIFORNIA              92653
       (ADDRESS OF PRINCIPAL EXECUTIVE OFFICES)                   (ZIP CODE)
</TABLE>

                                 (949) 462-9300
              (REGISTRANT'S TELEPHONE NUMBER, INCLUDING AREA CODE)

     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  ___

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<PAGE>   2

                                      INDEX

                        DEL TACO RESTAURANT PROPERTIES I

PART I.  FINANCIAL INFORMATION                                       PAGE NUMBER
------------------------------                                       -----------

Item 1.  Financial Statements and Supplementary Data

Balance Sheets at March 31, 2000 (Unaudited) and
     December 31, 1999                                                         3

Statements of Income for the three months ended
     March 31, 2000 and 1999 (Unaudited)                                       4

Statements of Cash Flows for the three months ended
     March 31, 2000 and 1999 (Unaudited)                                       5

Notes to Financial Statements                                                  6

Item 2.  Management's Discussion and Analysis of
         Financial Condition and Results of Operations                         8

PART II. OTHER INFORMATION

Item 6.  Exhibits and Reports on Form 8-K                                     10

SIGNATURES                                                                    11

                                      -2-


<PAGE>   3

                        DEL TACO RESTAURANT PROPERTIES I

                                 BALANCE SHEETS

<TABLE>
<CAPTION>
                                             MARCH 31,         December 31,
                                               2000               1999
                                             ---------         ------------
                                            (UNAUDITED)

                                     ASSETS
<S>                                         <C>               <C>
CURRENT ASSETS:
     Cash                                   $  164,344        $  170,290
     Receivable from General Partner            47,895            46,970
     Deposits                                      600               600
                                            ----------        ----------
        Total current assets                   212,839           217,860
                                            ----------        ----------


PROPERTY AND EQUIPMENT, AT COST:
     Land and improvements                   1,852,482         1,852,482
     Buildings and improvements              1,013,134         1,013,134
     Machinery and equipment                 1,136,026         1,136,026
                                            ----------        ----------
                                             4,001,642         4,001,642
     Less--accumulated depreciation          1,770,802         1,759,859
                                            ----------        ----------
                                             2,230,840         2,241,783
                                            ----------        ----------
                                            $2,443,679        $2,459,643
                                            ==========        ==========

                        LIABILITIES AND PARTNERS' EQUITY

CURRENT LIABILITIES:
    Payable to Limited Partners             $   39,295        $   31,001
    Accounts payable                            14,627             7,828
                                            ----------        ----------
        Total current liabilities               53,922            38,829
                                            ----------        ----------

PARTNERS' EQUITY:
    Limited Partners                         2,125,763         2,156,509
    General Partner-Del Taco, Inc.             263,994           264,305
                                            ----------        ----------
                                             2,389,757         2,420,814
                                            ----------        ----------
                                            $2,443,679        $2,459,643
                                            ==========        ==========
</TABLE>


                          The accompanying notes are an
                   integral part of these financial statements

                                      -3-


<PAGE>   4

                        DEL TACO RESTAURANT PROPERTIES I

                              STATEMENTS OF INCOME

                                   (UNAUDITED)

<TABLE>
<CAPTION>
                                          THREE MONTHS ENDED
                                                MARCH 31,
                                        -----------------------
                                         2000            1999
                                        -------        --------
<S>                                    <C>             <C>
REVENUES:
     Rent                              $134,251        $125,657
     Interest                               878           1,157
     Other                                  300             425
                                       --------        --------
                                        135,429         127,239
                                       --------        --------

EXPENSES:
     General and administrative          21,254          21,091
     Depreciation                        10,943          10,943
                                       --------        --------
                                         32,197          32,034
                                       --------        --------
        Net income                     $103,232        $ 95,205
                                       ========        ========
     Net income per limited
        partnership unit               $  11.68        $  10.77
                                       ========        ========
</TABLE>

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

                                      -4-


<PAGE>   5

                        DEL TACO RESTAURANT PROPERTIES I

                            STATEMENTS OF CASH FLOWS

                                   (UNAUDITED)



<TABLE>
<CAPTION>
                                                                       THREE MONTHS ENDED
                                                                            MARCH 31,
                                                                   ---------------------------
                                                                      2000             1999
                                                                   ---------         ---------
<S>                                                                <C>               <C>
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income                                                         $ 103,232         $  95,205
Adjustments to reconcile net income to net
  cash provided by operating activities:
     Depreciation                                                     10,943            10,943
     (Increase) decrease in receivable from General Partner             (925)            4,411
     Decrease in deposits                                                 --                30
     Increase in accounts payable and payable
        to limited partners                                           15,093             5,097
                                                                   ---------         ---------
           Net cash provided by operating activities                 128,343           115,686

CASH FLOWS FROM FINANCING ACTIVITIES:

Cash distributions to partners                                      (134,289)         (133,277)
                                                                   ---------         ---------
Net decrease in cash                                                  (5,946)          (17,591)
Beginning cash balance                                               170,290           162,979
                                                                   ---------         ---------
Ending cash balance                                                $ 164,344         $ 145,388
                                                                   =========         =========
</TABLE>

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

                                      -5-


<PAGE>   6

                        DEL TACO RESTAURANT PROPERTIES I

                          NOTES TO FINANCIAL STATEMENTS

                                 MARCH 31, 2000

NOTE 1 - BASIS OF PRESENTATION

The accompanying financial statements, some of which are unaudited, have been
prepared in accordance with generally accepted accounting principles for interim
financial information and with the instructions to Form 10-Q and Article 10 of
Regulation S-X. Accordingly, they do not include all of the information and
footnotes required by generally accepted accounting principles for complete
financial statements and should therefore be read in conjunction with the
financial statements and notes thereto contained in the Registrant's annual
report on Form 10-K for the year ended December 31, 1999. In the opinion of
management, all adjustments (consisting of normal recurring accruals) necessary
to present fairly the partnership's financial position at March 31, 2000, the
results of operations and cash flows for the three month periods ended March 31,
2000 and 1999 have been included. Operating results for the three months ended
March 31, 2000 are not necessarily indicative of the results that may be
expected for the year ending December 31, 2000.

NOTE 2 - NET INCOME PER LIMITED PARTNERSHIP UNIT

Net income per limited partnership unit is based upon the weighted average
number of units outstanding during the periods presented, which amounted to
8,751 in 2000 and 1999.

Pursuant to the partnership agreement, annual partnership net income is
allocated one percent to the General Partner and 99 percent to the limited
partners. A partnership net loss in any year will be allocated 24 percent to the
General Partner and 76 percent to the limited partners until the losses so
allocated equal income previously allocated. Any additional losses will be
allocated one percent to the General Partner and 99 percent to the limited
partners. Partnership gains from any sale or refinancing will be allocated one
percent to the General Partner and 99 percent to the limited partners until
allocated gains and profits equal losses. Additional gains will be allocated 24
percent to the General Partner and 76 percent to the limited partners.

                                      -6-


<PAGE>   7

                        DEL TACO RESTAURANT PROPERTIES I

                    NOTES TO FINANCIAL STATEMENTS - CONTINUED

                                 MARCH 31, 2000

NOTE 3 - LEASING ACTIVITIES

The partnership leases certain properties for operation of restaurants to Del
Taco, Inc. on a triple net basis. The leases are for terms of 35 years
commencing with the completion of the restaurant facility located on each
property and require monthly rentals equal to 12 percent of the gross sales of
the restaurants. There is no minimum rental under any of the leases.

For the three months ended March 31, 2000, the five restaurants operated by Del
Taco, for which the partnership is the lessor, had combined, unaudited sales of
$928,063 and net income of $43,758 as compared to $873,209 and $32,199
respectively, for the corresponding period in 1999. Net income by restaurant
includes charges for general and administrative expenses incurred in connection
with supervision of restaurant operations and interest expense (1999 combined,
unaudited net income amounts have been changed to measure net income on a
consistent basis with the year 2000 presentation). For the three months ended
March 31, 2000, the one restaurant operated by a Del Taco franchisee, for which
the partnership is the lessor, had unaudited sales of $190,697 as compared with
$173,933 during the same period in 1999.

For the three months ended March 31, 2000, the Elkhorn Boulevard restaurant in
Sacramento, California reported a net loss of $3,816 as compared to a net loss
of $3,883 for the corresponding period in 1999.

NOTE 4 - TRANSACTIONS WITH DEL TACO

The receivable from General Partner consists primarily of rent accrued for the
month of March. The March rent receivable was collected on April 10, 2000.

Del Taco, Inc. serves in the capacity of general partner in other partnerships
which are engaged in the business of operating restaurants, and three other
partnerships which were formed for the purpose of acquiring real property in
California for construction of Mexican-American restaurants for lease under
long-term agreements to Del Taco, Inc. for operation under the Del Taco trade
name.

In addition, see Note 5 with respect to certain distributions to the General
Partner.

NOTE 5 - DISTRIBUTIONS

On April 13, 2000, a distribution to the limited partners of $118,849, or
approximately $13.58 per limited partnership unit, was approved. Such
distribution was paid on April 19, 2000. The General Partner also received a
distribution of $1,200 with respect to its 1% partnership interest.

                                      -7-


<PAGE>   8

ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
        OF OPERATIONS

Liquidity and Capital Resources

The partnership offered limited partnership units for sale between March 1983
and March 1984. 15% of the $4.375 million raised through sale of limited
partnership units was used to pay commissions to brokers and to reimburse the
General Partner for offering costs incurred. Approximately $4 million of the
remaining funds were used to acquire sites and build six restaurants.

The six restaurants leased to Del Taco make up almost all of the income
producing assets of the partnership. Therefore, the business of the partnership
is almost entirely dependent on the success of the Del Taco trade name
restaurants that lease the properties. The success of the restaurants is
dependent on a large variety of factors, including, but not limited to, consumer
demand and preference for fast food, in general, and for Mexican-American food
in particular.

Results of Operations

The partnership owns six properties that are under long-term lease to Del Taco
for restaurant operations (Del Taco, in turn, has subleased two of the
restaurants to Del Taco franchisees, one of which is affiliated with Del Taco).

The following table sets forth rental revenue earned by restaurant:

                                              THREE MONTHS ENDED
                                                  MARCH 31,
                                             --------------------
                                               2000        1999
                                             --------    --------
Riverside Avenue, Rialto, CA                 $ 21,227    $ 20,157
Elden Avenue, Moreno Valley, CA                21,985      19,112
Foothill Boulevard, La Verne, CA               27,477      26,863
Baseline & Archibald, Rancho Cucamonga, CA     22,884      20,872
Elkhorn Boulevard, Sacramento, CA              14,379      13,837
Haven Avenue, Rancho Cucamonga, CA             26,299      24,816
                                             --------    --------
    Total                                    $134,251    $125,657
                                             ========    ========

The partnership receives rental revenues equal to 12 percent of gross sales from
the restaurants. The partnership earned rental revenue of $134,251 during the
three month period ended March 31, 2000, which represents an increase of $8,594
from 1999. The increase in rental revenue was caused by an increase in sales at
the restaurants under lease.

                                      -8-


<PAGE>   9

ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
        OF OPERATIONS - CONTINUED

The following table breaks down general and administrative expenses by type of
expense:

                                     Percentage of Total
                             General & Administrative Expense
                             --------------------------------
                                    Three Months Ended
                                         March 31,
                                    ------------------
                                    2000          1999
                                    ----          ----

Accounting fees                    70.81%        70.18%
Distribution of
  information to
  Limited Partners                 29.19         29.82
                                  ------        ------
                                  100.00%       100.00%
                                  ======        ======

General and administrative costs increased from 1999 to 2000 due to increased
costs for accounting and income tax return preparation.

Net income increased by $8,027 from 1999 to 2000 due to the increase in revenues
of $8,190 offset by the $163 increase in general and administrative expenses.

                                      -9-


<PAGE>   10

PART II.  OTHER INFORMATION

Item 6.  Exhibits and Reports on Form 8-K

(b) No reports on Form 8-K were filed during the three months ended March 31,
2000.

                                      -10-


<PAGE>   11

                                   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.

                                 DEL TACO RESTAURANT PROPERTIES I
                                 (a California limited partnership)
                                 Registrant

                                 Del Taco, Inc.
                                 General Partner


Date:  May 1, 2000              /s/ Robert J. Terrano
                                ----------------------------
                                    Robert J. Terrano
                                    Executive Vice President,
                                    Chief Financial Officer


Date:  May 1, 2000              /s/ C. Douglas Mitchell
                                ----------------------------
                                    C. Douglas Mitchell
                                    Vice President and Corporate
                                    Controller

                                      -11-


<PAGE>   12

                                 EXHIBIT INDEX

EXHIBIT
NUMBER                 DESCRIPTION
-------                -----------

  27                   Financial Data Schedule