<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                SEPTEMBER 30, 1999               .

                                       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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                                      INDEX

                        DEL TACO RESTAURANT PROPERTIES I


<TABLE>
<CAPTION>
PART I.  FINANCIAL INFORMATION                                       PAGE NUMBER
------------------------------                                       -----------
<S>                                                                  <C>

Item 1.  Financial Statements and Supplementary Data


Balance Sheets at September 30, 1999 (Unaudited) and
     December 31, 1998                                                  3

Statements of Income for the three and nine months ended
     September 30, 1999 and 1998 (Unaudited)                            4

Statements of Cash Flows for the nine months ended
     September 30, 1999 and 1998 (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
</TABLE>



                                      -2-
<PAGE>   3

                        DEL TACO RESTAURANT PROPERTIES I

                                 BALANCE SHEETS


<TABLE>
<CAPTION>
                                                    SEPTEMBER 30,     December 31,
                                                        1999              1998
                                                    -------------     ------------
                                                     (UNAUDITED)
<S>                                                 <C>               <C>
                                     ASSETS

CURRENT ASSETS:
     Cash                                            $  165,285        $  162,979
     Receivable from General Partner (Note 4)            45,654            47,833
     Deposits                                               684               797
                                                     ----------        ----------
        Total current assets                            211,623           211,609
                                                     ----------        ----------


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                   1,748,916         1,716,087
                                                     ----------        ----------
                                                      2,252,726         2,285,555
                                                     ----------        ----------

                                                     $2,464,349        $2,497,164
                                                     ==========        ==========

                        LIABILITIES AND PARTNERS' EQUITY

CURRENT LIABILITIES:
     Payable to Limited Partners                     $   26,681        $   24,702
     Accounts payable                                     3,976             5,703
                                                     ----------        ----------
        Total current liabilities                        30,657            30,405
                                                     ----------        ----------


PARTNERS' EQUITY:
     Limited Partners                                 2,169,227         2,201,963
     General Partner-Del Taco, Inc.                     264,465           264,796
                                                     ----------        ----------
                                                      2,433,692         2,466,759
                                                     ----------        ----------

                                                     $2,464,349        $2,497,164
                                                     ==========        ==========
</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             NINE MONTHS ENDED
                                                SEPTEMBER 30,                  SEPTEMBER 30,
                                           1999            1998            1999            1998
                                         --------        --------        --------        --------
<S>                                      <C>             <C>             <C>             <C>
REVENUES:
     Rent (Notes 3 and 4)                $141,151        $141,492        $397,688        $392,023
     Interest                               1,222           1,193           3,304           2,378
     Other                                    353             650             953           1,274
                                         --------        --------        --------        --------
                                          142,726         143,335         401,945         395,675
                                         --------        --------        --------        --------

EXPENSES:
     General and administrative             7,199           7,498          38,225          38,122
     Depreciation                          10,943          10,943          32,829          32,829
                                         --------        --------        --------        --------
                                           18,142          18,441          71,054          70,951
                                         --------        --------        --------        --------

        Net income                       $124,584        $124,894        $330,891        $324,724
                                         ========        ========        ========        ========

     Net income per limited
        partnership unit (Note 3)        $  14.09        $  14.13        $  37.43        $  36.74
                                         ========        ========        ========        ========
</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>
                                                                        NINE MONTHS ENDED
                                                                          SEPTEMBER 30,
                                                                     1999              1998
                                                                   ---------         ---------
<S>                                                                <C>               <C>
CASH FLOWS FROM OPERATING ACTIVITIES:

Net income                                                         $ 330,891         $ 324,724
Adjustments to reconcile net income to net
  cash provided by operating activities:
     Depreciation                                                     32,829            32,829
     Decrease (increase) in receivable from General Partner            2,179            (2,832)
     Decrease (increase) in deposits                                     113              (394)
     Increase in accounts payable and payable
        to limited partners                                              252             5,035
                                                                   ---------         ---------

           Net cash provided by operating activities                 366,264           359,362


CASH FLOWS FROM FINANCING ACTIVITIES:

Cash distributions to partners                                      (363,958)         (339,094)
                                                                   ---------         ---------

Net increase in cash                                                   2,306            20,268

Beginning cash balance                                               162,979           143,280
                                                                   ---------         ---------

Ending cash balance                                                $ 165,285         $ 163,548
                                                                   =========         =========
</TABLE>



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



                                      -5-
<PAGE>   6

                        DEL TACO RESTAURANT PROPERTIES I

                          NOTES TO FINANCIAL STATEMENTS

                               SEPTEMBER 30, 1999


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, 1998. In the opinion of
management, all adjustments (consisting of normal recurring accruals) necessary
to present fairly the partnership's financial position at September 30, 1999,
the results of operations and cash flows for the nine month periods ended
September 30, 1999 and 1998 have been included. Operating results for the three
and nine months ended September 30, 1999 are not necessarily indicative of the
results that may be expected for the year ending December 31, 1999.

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 1999 and 1998.

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

                               SEPTEMBER 30, 1999


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 September 30, 1999, the five restaurants operated by
Del Taco, for which the partnership is the lessor, had combined, unaudited sales
of $977,544 and net income of $61,546 as compared to $991,832 and $63,716
respectively, for the corresponding period in 1998. Net income by restaurant
includes charges for general and administrative expenses incurred in connection
with supervision of restaurant operations and interest expense. For the three
months ended September 30, 1999, the one restaurant operated by a Del Taco
franchisee, for which the partnership is the lessor, had unaudited sales of
$198,717 as compared with $187,269 during the same period in 1998.

For the nine months ended September 30, 1999, the five restaurants operated by
Del Taco, for which the partnership is the lessor, had combined, unaudited sales
of $2,756,556 and net income of $140,234 as compared to $2,753,257 and $145,627
respectively, for the corresponding period in 1998. For the nine months ended
September 30, 1999, the one restaurant operated by a Del Taco franchisee, for
which the partnership is the lessor, had unaudited sales of $557,517 as compared
with $513,602 during the same period in 1998.

For the three months and nine months ended September 30, 1999, the Elkhorn
Boulevard restaurant in Sacramento, California reported net losses of $1,986 and
$6,927 as compared to net losses of $6,054 and $14,778 for the corresponding
periods in 1998.

NOTE 4 - TRANSACTIONS WITH DEL TACO

The receivable from General Partner consists primarily of rent accrued for the
month of September. The September rent receivable was collected on October 13,
1999.

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 October 12, 1999, a distribution to the limited partners of $132,268, or
approximately $15.11 per limited partnership unit, was approved. Such
distribution was paid on October 19, 1999. The General Partner also received a
distribution of $1,367 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 for the year:

<TABLE>
<CAPTION>
                                                  THREE MONTHS ENDED          NINE MONTHS ENDED
                                                     SEPTEMBER 30,               SEPTEMBER 30,
                                                  1999          1998          1999          1998
                                                --------      --------      --------      --------
<S>                                             <C>           <C>           <C>           <C>
Riverside Avenue, Rialto, CA                    $ 22,671      $ 22,814      $ 64,123      $ 63,832

Elden Avenue, Moreno Valley, CA                   20,736        21,970        59,543        60,585

Foothill Boulevard, La Verne, CA                  30,650        30,386        86,111        84,532

Baseline & Archibald, Rancho Cucamonga, CA        23,846        22,472        66,902        61,632

Elkhorn Boulevard, Sacramento, CA                 16,233        14,691        45,140        39,974

Haven Avenue, Rancho Cucamonga, CA                27,015        29,159        75,869        81,468
                                                --------      --------      --------      --------

        Total                                   $141,151      $141,492      $397,688      $392,023
                                                ========      ========      ========      ========
</TABLE>


The partnership receives rental revenues equal to 12 percent of gross sales from
the restaurants. The partnership earned rental revenue of $141,151 during the
three month period ended September 30, 1999, which represents a decrease of $341
from 1998. The partnership earned rental revenue of $397,688 during the nine
month period ended September 30, 1999, which represents an increase of $5,665
from 1998. The changes in rental revenue between 1998 and 1999 are directly
attributable to changes in sales levels 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:


<TABLE>
<CAPTION>
                                                     Percentage of Total
                                               General & Administrative Expense
                                               --------------------------------

                                                      Nine Months Ended
                                                        September 30,
                                                    1999           1998
                                                   -------        -------
<S>                                            <C>                <C>
Accounting fees                                      67.68%         68.32%
Distribution of
  information to
  Limited Partners                                   32.32          31.68
                                                   -------        -------

                                                    100.00%        100.00%
                                                   =======        =======
</TABLE>

General and administrative costs for the nine month period ended September 30,
increased from 1998 to 1999 due to increased costs for accounting and income tax
return preparation.

For the three month period ended September 30, 1999, net income decreased $310
from 1998 to 1999 due to the decrease in revenues of $609 which was partially
offset by the $299 decrease in general and administrative expenses. For the nine
month period ended September 30, 1999, net income increased by $6,167 from 1998
to 1999 due to the increase in revenues of $6,270 which was partially offset by
the $103 increase in general and administrative expenses.

The General Partner does not believe the operations of the partnership will be
significantly impacted by the year 2000 software issue and does not believe the
year 2000 software issue will materially effect the partnerships operations,
financial position or cash flows.



                                      -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 nine months ended September 30,
1999.



                                      -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:  October 29, 1999                 /s/ Robert J. Terrano
                                        ----------------------------
                                        Robert J. Terrano
                                        Executive Vice President,
                                        Chief Financial Officer


Date:  October 29, 1999                 /s/ C. Douglas Mitchell
                                        ----------------------------
                                        C. Douglas Mitchell
                                        Vice President and Corporate
                                        Controller



                                      -11-
<PAGE>   12
                                 EXHIBIT INDEX

<TABLE>
<S>       <C>
     27   Financial Data Schedule
</TABLE>