<PAGE>   1

                                  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, 1998
                              --------------------------------
                                          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)

                 CALIFORNIA                         95-3852699
       (State or other jurisdiction of            (I.R.S. Employer
        incorporation or organization)          Identification Number)

      23041 AVENIDA DE LA CARLOTA, SUITE 400, LAGUNA HILLS, CA      92653
        (Address of principal executive offices)                 (Zip Code)

                                 (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 [ ]



                                      -1-
<PAGE>   2

                                      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, 1998 (Unaudited) and
     December 31, 1997                                                   3

Statements of Income for the three and nine months ended
     September 30, 1998 and 1997 (Unaudited)                             4

Statements of Cash Flows for the nine months ended
     September 30, 1998 and 1997 (Unaudited)                             5

Notes to Financial Statements                                            6


Item 2.  Management's Discussion and Analysis of
         Financial Condition and Results of Operations                   9


PART II. OTHER INFORMATION

Item 6.  Exhibits and Reports on Form 8-K                               11


SIGNATURES                                                              12
</TABLE>



                                      -2-
<PAGE>   3

                        DEL TACO RESTAURANT PROPERTIES I

                                 BALANCE SHEETS


<TABLE>
<CAPTION>
                                                   SEPTEMBER 30         December 31
                                                       1998                1997
                                                    ----------          ----------
                                                   (UNAUDITED)
<S>                                                <C>                  <C>       
                                     ASSETS
CURRENT ASSETS:
  Cash                                              $  163,548          $  143,280
  Receivable from General Partner (Note 4)              45,433              42,601
  Deposits                                                 994                 600
                                                    ----------          ----------
    Total current assets                               209,975             186,481
                                                    ----------          ----------

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                     1,705,144           1,672,315
                                                    ----------          ----------
                                                     2,296,498           2,329,327
                                                    ----------          ----------
                                                    $2,506,473          $2,515,808
                                                    ==========          ==========

                        LIABILITIES AND PARTNERS' EQUITY


CURRENT LIABILITIES:
  Payable to Limited Partners                       $   21,574          $   12,690
  Accounts payable                                       2,683               6,531
                                                    ----------          ----------
     Total current liabilities                          24,257              19,221
                                                    ----------          ----------

PARTNERS' EQUITY:
  Limited Partners                                   2,217,407           2,231,634
  General Partner-Del Taco, Inc.                       264,809             264,953
                                                    ----------          ----------
                                                     2,482,216           2,496,587
                                                    ----------          ----------

                                                    $2,506,473          $2,515,808
                                                    ==========          ==========
</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
                                      1998            1997            1998            1997 
                                    --------        --------        --------        --------
<S>                                 <C>             <C>             <C>             <C>     
REVENUES:
  Rent (Notes 3 and 4)              $141,492        $126,241        $392,023        $359,312
  Interest                             1,193             814           2,378           2,595
  Other                                  650           1,125           1,274           1,600
                                    --------        --------        --------        --------
                                     143,335         128,180         395,675         363,507
                                    --------        --------        --------        --------

EXPENSES:
  General and administrative           7,498           5,664          38,122          35,231
  Depreciation                        10,943          10,943          32,829          32,829
                                    --------        --------        --------        --------
                                      18,441          16,607          70,951          68,060
                                    --------        --------        --------        --------

Net income                          $124,894        $111,573        $324,724        $295,447
                                    ========        ========        ========        ========

Net income per Limited
  Partnership Unit (Note 2)         $  14.13        $  12.62        $  36.74        $  33.42
                                    ========        ========        ========        ========
</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
                                                           1998                1997 
                                                         ---------           ---------
<S>                                                      <C>                 <C>      
CASH FLOWS FROM OPERATING ACTIVITIES:

 Net income                                              $ 324,724           $ 295,447
 Adjustments to reconcile net income to net
  cash provided by operating activities:
    Depreciation                                            32,829              32,829
    Increase in payable to Limited Partners                  8,883                 885
    Increase in receivable from
     General Partner                                        (2,832)             (5,964)
    Decrease in accounts payable                            (3,848)             (1,366)
    Increase in deposits                                      (394)                 --
                                                         ---------           ---------

      Net cash provided by operating activities            359,362             321,831


CASH FLOWS FROM FINANCING ACTIVITIES:

 Cash distributions to partners                           (339,094)           (313,075)
                                                         ---------           ---------

Net increase in cash                                        20,268               8,756

Beginning cash balance                                     143,280             130,996
                                                         ---------           ---------

Ending cash balance                                      $ 163,548           $ 139,752
                                                         =========           =========
</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, 1998


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, 1997. In the opinion of
management, all adjustments (consisting of normal recurring accruals) necessary
to present fairly the partnership's financial position at September 30, 1998,
the results of operations and cash flows for the nine month periods ended
September 30, 1998 and 1997 have been included. Operating results for the three
and nine months ended September 30, 1998 are not necessarily indicative of the
results that may be expected for the year ending December 31, 1998.

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 1998 and 1997.

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, 1998


NOTE 3 - LEASING ACTIVITIES

The Registrant leases (the "Leases") certain properties (the "Properties") for
operation of restaurants to Del Taco, Inc. ("General Partner") 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. The Registrant had a total of six Properties
leased as of September 30, 1998 and 1997, two of which have been subleased to
Del Taco franchisees (one of which is affiliated with Del Taco, Inc.)

For the three months ended September 30, 1998, the five restaurants operated by
Del Taco, for which the Registrant is the lessor, had combined, unaudited sales
of $991,832 and net income of $63,716 as compared to $891,056 and $36,149
respectively, for the corresponding period in 1997. Net income by restaurant
includes charges for general & administrative expenses incurred in connection
with supervision of restaurant operations and interest expense. For the three
months ended September 30, 1998, the one restaurant operated by a Del Taco
franchisee, for which the Registrant is the lessor, had unaudited sales of
$187,269 as compared with $160,958 during the same period in 1997.

For the nine months ended September 30, 1998, the five restaurants operated by
Del Taco, for which the Registrant is the lessor, had combined, unaudited sales
of $2,753,257 and net income of $145,627 as compared to $2,549,509 and $87,122
respectively, for the corresponding period in 1997. For the nine months ended
September 30, 1998, the one restaurant operated by a Del Taco franchisee, for
which the Registrant is the lessor, had unaudited sales of $513,602 as compared
with $444,767 during the same period in 1997.

For the three months and nine months ended September 30, 1998, the Elkhorn
Boulevard restaurant in Sacramento, California reported net losses of $6,054 and
$14,778 as compared to net losses of $2,619 and $12,579 respectively, for the
corresponding period in 1997.



                                      -7-
<PAGE>   8

DEL TACO RESTAURANT PROPERTIES I
NOTES TO FINANCIAL STATEMENTS - CONTINUED
SEPTEMBER 30, 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,
1998.

Del Taco, Inc. serves in the capacity of general partner in other partnerships
which are engaged in the business of operating restaurants, and four
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, 1998, a distribution to the Limited Partners of $135,365 or
approximately $15.47 per Limited Partnership Unit, was approved. Such
distribution was paid on October 15, 1998. The General Partner also received a
distribution of $1,367 with respect to its 1% partnership interest.



                                      -8-
<PAGE>   9

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

Liquidity and Capital Resources

The Registrant commenced offering of Limited Partnership Units on March 21,
1983. By March 20, 1984, the sale of such Units provided a total capitalization
for the Registrant of $4,375,500. Fifteen percent of the cash received from the
sale of Limited Partnership Units was used to pay commissions to brokers and to
reimburse the General Partner for offering costs incurred. The remaining funds
were expended for the acquisition of sites and construction of six restaurants.
In 1985, the first four restaurants opened for business and two additional
restaurants were opened in 1986. Approximately $4,002,000 was expended for such
purposes.

Since the six restaurants owned by the Registrant opened, cash flow from Lease
payments received from Del Taco, the Registrant's General Partner, which leases
all six restaurants, has provided adequate liquidity for operation of the
Registrant. However, the Registrant's overwhelmingly predominant source of
income to meet its expenses and fund distributions to its Limited Partners is
payments from Del Taco under the Leases, comprising primarily rent calculated on
the basis of the gross sales of the restaurants operated on the Properties, as
to which there are no contractually specified minimum or guaranteed amounts.
Thus, the adequacy of the Registrant's liquidity and capital resources in the
future will depend primarily upon the gross revenues of such restaurants as well
as upon Del Taco's financial condition and results of operations generally.

Results of Operations

The Registrant owns six Mexican-American restaurants that are under long-term
lease to its General Partner for operation under the Del Taco trade name. The
Registrant receives rental revenues equal to 12 percent of restaurant sales. For
the three months ended September 30, rental revenues increased from $126,241 in
1997 to $141,492 in 1998. For the nine months ended September 30, rental
revenues increased from $359,312 1997 to $392,023 in 1998. The increases in
rental revenue are directly attributable to increases in sales at the
restaurants.



                                      -9-
<PAGE>   10

The following table sets forth rental revenue earned by restaurant for the
quarter and year to date:

<TABLE>
<CAPTION>
                                            THREE MONTHS ENDED               NINE MONTHS ENDED
                                                SEPTEMBER 30                   SEPTEMBER 30
                                           1998            1997            1998            1997
                                         --------        --------        --------        --------
<S>                                      <C>             <C>             <C>             <C>     
Riverside Avenue, Rialto, CA             $ 22,814        $ 21,458        $ 63,832        $ 59,580

Elden Avenue, Moreno Valley, CA            21,970          18,558          60,585          55,331

Foothill Boulevard, La Verne, CA           30,386          26,865          84,532          77,744

Baseline & Archibald, Rancho               22,472          19,315          61,632          53,372
  Cucamonga, CA

Elkhorn Boulevard, Sacramento, CA          14,691          12,957          39,974          35,911

Haven Avenue, Rancho                       29,159          27,088          81,468          77,374
                                         --------        --------        --------        --------
  Cucamonga, CA

       Total                             $141,492        $126,241        $392,023        $359,312
                                         ========        ========        ========        ========
</TABLE>


The following table sets forth the percentage relationship to total general and
administrative expenses of items included in the Registrant's Statements of
Income:

                              Percentage of Total
                        General & Administrative Expense

<TABLE>
<CAPTION>
                                                          Nine Months Ended
                                                            September 30
                                                        1998              1997 
                                                      -------           -------
<S>                                                   <C>               <C>   
Accounting fees                                         60.53%            59.61%
Distribution of
  information to
  Limited Partners                                      37.37             38.12
Other                                                    2.10              2.27
                                                      -------           -------
                                                       100.00%           100.00%
                                                      =======           =======
</TABLE>

Certain reclassifications have been made to the fiscal year 1997 general and
administrative expenses to conform to the fiscal year 1998 presentation.
Operating expenses include general and administrative expenses which consist
primarily of accounting fees and costs of distribution of information to the
Limited Partners. For the three months ended September 30, general and
administrative expenses increased from $5,664 in 1997 to $7,498 in 1998. For the
nine months ended September 30, general and administrative expenses increased
from $35,231 in 1997 to $38,122 in 1998. General and administrative expenses
increased due to increased costs of printing and distribution. For the three
months ended September 30, the Registrant incurred depreciation expense in the
amount of $10,943 in 1998 and 1997. The Registrant incurred depreciation expense
of $32,829 for nine months ended September 30, 1998 and 1997,respectively.



                                      -10-
<PAGE>   11

For the three months ended September 30, revenues increased by $15,155 while
expenses increased by $1,834 resulting in an increase in net income from
$111,573 in 1997 to $124,894 in 1998. For the nine months ended September 30,
revenues increased by $32,168 while expenses increased by $2,891 resulting in an
increase in the Registrant's net income from $295,447 for the nine months ended
September 30, 1997 to $324,724 for the corresponding period in 1998.

For the reasons stated under "Liquidity and Capital Resources" above, the
Registrant's results of operations in the future will depend primarily upon the
gross revenues of the restaurants located on the Properties leased to Del Taco
as well as upon Del Taco's financial condition and results of operations
generally.

Management does not believe the operations of the Company will be significantly
impacted by the year 2000 software issue and does not believe the year 2000
software issue will materially effect the Company's operations, financial
position or cash flows.


PART II.  OTHER INFORMATION

Item 6. Exhibits and Reports on Form 8-K

(a) Exhibit 27 - Financial Data Schedule.

(b) No reports on Form 8-K were filed during the nine months ended 
    September 30, 1998.



                                      -11-
<PAGE>   12

                                   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 30, 1998                      /s/ Robert J. Terrano       
                                             -----------------------------------
                                             Robert J. Terrano
                                             Executive Vice President,
                                             Chief Financial Officer


Date:  October 30, 1998                      /s/ C. Douglas Mitchell     
                                             -----------------------------------
                                             C. Douglas Mitchell
                                             Vice President and Corporate
                                             Controller



                                      -12-
<PAGE>   13

                                 EXHIBIT INDEX


Exhibit
Number                             Description
------                             -----------

  27                         Financial Data Schedule