<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,                  MARCH 31, 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)
 
<TABLE>
<S>                                                          <C>
                         CALIFORNIA                                    95-3852699
              (STATE OR OTHER JURISDICTION OF                       (I.R.S. EMPLOYER
               INCORPORATION OR ORGANIZATION)                      IDENTIFICATION NO.)
 
   23041 AVENIDA DE LA CARLOTA, LAGUNA HILLS, CALIFORNIA                  92653
          (ADDRESS OF PRINCIPAL EXECUTIVE OFFICES)                     (ZIP CODE)
</TABLE>
 
                                 (714) 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  ___
 
================================================================================
<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, 1998 (Unaudited) and
     December 31, 1997                                           3

Statements of Income for the three months ended
     March 31, 1998 and 1997 (Unaudited)                         4

Statements of Cash Flows for the three months ended
     March 31, 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


<PAGE>   3

                        DEL TACO RESTAURANT PROPERTIES I

                                 BALANCE SHEETS

<TABLE>
<CAPTION>
                                                                 MARCH 31,         December 31,
                                                                   1998               1997
                                                                -----------        ------------
                                                                (UNAUDITED)
                                        ASSETS
<S>                                                             <C>                <C>
CURRENT ASSETS:
  Cash                                                          $  113,304         $  143,280
  Receivable from General Partner (Note 4)                          43,346             42,601
  Deposits                                                             600                600
                                                                ----------         ----------
    Total current assets                                           157,250            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                                 1,683,258          1,672,315
                                                                ----------         ----------
                                                                 2,318,384          2,329,327
                                                                ----------         ----------

                                                                $2,475,634         $2,515,808
                                                                ==========         ==========

                        LIABILITIES AND PARTNERS' EQUITY

CURRENT LIABILITIES:
  Payable to Limited Partners                                   $   12,690         $   12,690
  Accounts payable                                                   3,101              6,531
                                                                ----------         ----------
     Total current liabilities                                      15,791             19,221
                                                                ----------         ----------

PARTNERS' EQUITY:
  Limited Partners                                               2,195,257          2,231,634
  General Partner-Del Taco, Inc.                                   264,586            264,953
                                                                ----------         ----------
                                                                 2,459,843          2,496,587
                                                                ----------         ----------

                                                                $2,475,634         $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
                                                                            MARCH 31
                                                                    1998                 1997
                                                                  ----------------------------
<S>                                                               <C>                 <C>
REVENUES:
  Rent (Notes 3 and 4)                                            $119,375            $110,917
  Interest                                                             612               1,024
  Other                                                                449                  75
                                                                  --------            --------
                                                                   120,436             112,016
                                                                  --------            --------

EXPENSES:
  General and administrative                                        20,647              20,807
  Depreciation                                                      10,943              10,943
                                                                  --------            --------
                                                                    31,590              31,750
                                                                  --------            --------

Net income                                                        $ 88,846            $ 80,266
                                                                  ========            ========

Net income per Limited
  Partnership Unit (Note 2)                                       $  10.05            $   9.08
                                                                  ========            ========
</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
                                                                    1998               1997
                                                                  --------          ---------
<S>                                                               <C>               <C>
CASH FLOWS FROM OPERATING ACTIVITIES:

Net income                                                        $ 88,846          $  80,266
Adjustments to reconcile net income to net
  cash provided by operating activities:
    Depreciation                                                    10,943             10,943
    Increase in receivable from General Partner                       (745)            (4,319)
    Increase (decrease) in accounts payable                         (3,430)             1,265
                                                                  --------           --------

      Net cash provided by operating activities                     95,614             88,155


CASH FLOWS FROM FINANCING ACTIVITIES:

 Cash distributions to partners                                   (125,590)          (111,677)
                                                                  --------           --------

Net decrease in cash                                               (29,976)           (23,522)

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

Ending cash balance                                               $113,304           $107,474
                                                                  ========           ========
</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, 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 March 31, 1998, the
results of operations and cash flows for the three month periods ended March 31,
1998 and 1997 have been included. Operating results for the three months ended
March 31, 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

                                 MARCH 31, 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 March 31, 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 March 31, 1998, the five restaurants operated by Del
Taco, for which the Registrant is the lessor, had combined, unaudited sales of
$836,260 and net income of $30,700 as compared to $790,700 and $21,832
respectively, for the corresponding period in 1997. 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 March 31, 1998, the one restaurant operated by a Del Taco
franchisee, for which the Registrant is the lessor, had unaudited sales of
$158,530 as compared with $133,608 during the same period in 1997.

For the three months ended March 31, 1998, the Elkhorn Boulevard restaurant in
Sacramento, California reported a net loss of $4,124 as compared to a net loss
of $4,566 for the corresponding period in 1997.

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, 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.


                                      -7-


<PAGE>   8

                        DEL TACO RESTAURANT PROPERTIES I

                    NOTES TO FINANCIAL STATEMENTS - CONTINUED

                                 MARCH 31, 1998


NOTE 5 - DISTRIBUTIONS

On April 10, 1998, a distribution to the Limited Partners of $94,658, or
approximately $10.82 per Limited Partnership Unit, was approved. Such
distribution was paid on April 24, 1998. The General Partner also received a
distribution of $956 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 March 31, rental revenues increased from $110,917 in 1997
to $119,375 in 1998. Such increases are directly attributable to increased sales
at the restaurants.

The following table sets forth rental revenue earned by restaurant for the
quarter:


                                      -9-

<PAGE>   10

<TABLE>
<CAPTION>
                                                                       THREE MONTHS ENDED
                                                                            MARCH 31
                                                                     1998              1997
                                                                   --------          --------
<S>                                                                <C>               <C>
Riverside Avenue, Rialto, CA                                       $ 19,456          $ 17,570

Elden Avenue, Moreno Valley, CA                                      17,948            17,342

Foothill Boulevard, La Verne, CA                                     25,806            24,774

Baseline & Archibald, Rancho Cucamonga, CA                           19,024            16,033

Elkhorn Boulevard, Sacramento, CA                                    12,014            10,574

Haven Avenue, Rancho Cucamonga, CA                                   25,127            24,624
                                                                   --------          --------

                                    Total                          $119,375          $110,917
                                                                   ========          ========
</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:

<TABLE>
<CAPTION>
                                               Percentage of Total
                                         General & Administrative Expense
                                         --------------------------------
                                               Three Months Ended
                                                    March 31
                                              1998           1997
                                              -----          -----
<S>                                           <C>           <C>
Accounting fees                               69.26%         76.39%
Distribution of
  information to
  Limited Partners                            30.74          23.61
                                             ------         ------

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

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 March 31, general and
administrative expenses decreased from $20,807 in 1997 to $20,647 in 1998. For
the three months ended March 31, the Registrant incurred depreciation expense in
the amount of $10,943 in 1998 and 1997.

For the three months ended March 31, revenues increased by $8,420 while expenses
decreased by $160 resulting in an increase in net income from $80,266 in 1997 to
$88,846 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.


                                      -10-

<PAGE>   11
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 three months ended 
    March 31, 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:  April 30, 1998                       /s/ Robert J. Terrano
                                            -----------------------------------
                                            Robert J. Terrano
                                            Executive Vice President,
                                            Chief Financial Officer


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


                                      -12-