<PAGE>   1
 
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                                 UNITED STATES
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
 
                                   FORM 10-K
 
(MARK ONE)

[X]   ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE
      SECURITIES EXCHANGE ACT OF 1934 [FEE REQUIRED]
 
      FOR THE FISCAL YEAR ENDED DECEMBER 31, 1998
 
                                       OR
 
[ ]   TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE
      SECURITIES EXCHANGE ACT OF 1934 [NO FEE REQUIRED]
 
      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>
 
       Registrant's telephone number, including area code: (949) 462-9300
 
        Securities registered pursuant to section 12(b) of the Act: None
 
        Securities registered pursuant to section 12(g) of the Act: None
 
     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 ___
 
                      DOCUMENTS INCORPORATED BY REFERENCE
 
     Portions of the registrant's Form S-11 Registration Statement filed
December 17, 1982 are incorporated by reference into Part IV of this report.
 
--------------------------------------------------------------------------------
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<PAGE>   2

                                     PART I

ITEM 1. BUSINESS

The partnership is a publicly-held limited partnership organized under the
California Uniform Limited Partnership Act. The partnership's General Partner is
Del Taco, Inc., a California corporation ("General Partner"). The partnership
sold 8,751 units totaling $4.375 million through an offering of limited
partnership units from March 1983 through March 1984. The term of the
partnership agreement is until April 30, 2022 unless terminated earlier by means
provided in the partnership agreement.

The business of the partnership is ownership and leasing of restaurants in
California to Del Taco, Inc. The partnership acquired land and constructed six
Mexican-American restaurants for long-term lease to Del Taco, Inc. Each property
is leased for 35 years on a triple net basis. Rent is equal to twelve percent of
gross sales of the restaurants. As of December 31, 1998, the partnership had a
total of six properties leased to Del Taco (Del Taco, in turn, has subleased two
of the restaurants).

The partnership has no full time employees. The partnership agreement assigns
full authority for general management and supervision of the business affairs of
the partnership to the General Partner. The General Partner has a one percent
interest in the profits or losses and distributions of the partnership. Limited
partners have no right to participate in the management or conduct of the
partnership's business affairs.



                                       2

<PAGE>   3

ITEM 2. PROPERTIES

The partnership has acquired six properties with proceeds obtained from the sale
of partnership units:

<TABLE>
<CAPTION>
                                              Date of              Restaurant             Date of Commencement
Address                City, State            Acquisition          Constructed            of Operation (1)
-------                -----------            -----------          -----------            --------------------

<S>                    <C>                    <C>                  <C>                    <C> 
Riverside Avenue       Rialto, CA             September 28, 1984   60 seat with drive     February 12, 1985
                                                                   through service
                                                                   window

Elden Avenue           Moreno Valley, CA      March 8, 1985        60 seat with drive     June 30, 1985
                                                                   through service        
                                                                   window

Foothill Boulevard     La Verne, CA           April 16, 1985       60 seat with drive     November 6, 1985
                                                                   through service    
                                                                   window

Baseline & Archibald   Rancho Cucamonga, CA   July 10, 1985        60 seat with drive     November 26, 1985
                                                                   through service
                                                                   window

Elkhorn Boulevard      Sacramento, CA         August 22, 1985      60 seat with drive     January 15, 1986
                                                                   through service  
                                                                   window

Haven Avenue           Rancho Cucamonga, CA   September 20, 1985   60 seat with drive     February 14, 1986
                                                                   through service
                                                                   window
</TABLE>

-------------
(1) Commencement of operation is the first date Del Taco, Inc., as lessee,
    operated the facility on the site as a Del Taco restaurant.


                                       3

<PAGE>   4

                                     PART II

ITEM 3. LEGAL PROCEEDINGS

The partnership is not a party to any material pending legal proceedings.

ITEM 4. SUBMISSIONS OF MATTERS TO A VOTE OF SECURITY HOLDERS

None.

ITEM 5. MARKET FOR THE PARTNERSHIP'S COMMON EQUITY AND RELATED SECURITY HOLDER
        MATTERS

The partnership sold 8,751 ($4,375,500) limited partnership units during the
public offering period ended March 20, 1984 and currently has 870 limited
partners of record. There is no public market for the trading of the units.
Distributions made by the partnership to the limited partners during the past
three fiscal years are described in Note 6 to the Notes to the Financial
Statements contained under Item 8.




                                       4

<PAGE>   5

ITEM 6. SELECTED FINANCIAL DATA

<TABLE>
<CAPTION>
                                                         For the Year Ended December 31,
                                    ------------------------------------------------------------------------
                                      1998            1997            1996            1995            1994
                                    --------        --------        --------        --------        --------
<S>                                 <C>             <C>             <C>             <C>             <C>     
Rental revenue                      $531,608        $487,280        $452,203        $436,262        $420,245

Interest and                           4,924           5,109           2,902           2,729           2,729
  other income

Net income                           445,860         403,356         365,387         275,020         222,295

Net income
  per limited
  partnership
  unit (1)                             50.44           45.63           41.34           31.11           25.15

Cash distributions
  per limited
  partnership
  unit (2)(3)(4)(5)(6)                 53.84           49.28           44.82          45.70            58.70

Total assets                       2,497,164       2,515,808       2,543,173       2,569,140       2,700,433

Long-term obligations                   None            None            None            None            None
</TABLE>

--------------
(1) The net income per limited partnership unit was calculated based upon 8,751
    weighted average units outstanding for all years presented.

(2) Cash distributions for the quarter ended December 31, 1994 amounted to
    $11.70 per limited partnership unit and were paid January 17, 1995. Five
    distributions were disbursed during the year ended December 31, 1994.


                                       5

<PAGE>   6

ITEM 6. SELECTED FINANCIAL DATA - CONTINUED

(3) Cash distributions for the quarter ended December 31, 1995 amounted to
    $11.46 per limited partnership unit and were paid January 15, 1996. Four
    distributions were disbursed during the year ended December 31, 1995.

(4) Cash distributions for the quarter ended December 31, 1996 amounted to
    $12.63 per limited partnership unit and were paid January 31, 1997. Four
    distributions were disbursed during the year ended December 31, 1996.

(5) Cash distributions for the quarter ended December 31, 1997 amounted to
    $14.21 per limited partnership unit and were paid January 31, 1998. Four
    distributions were disbursed during the year ended December 31, 1997.

(6) Cash distributions for the quarter ended December 31, 1998 amounted to
    $15.08 per limited partnership unit and were paid January 31, 1999. Four
    distributions were disbursed during the year ended December 31, 1998.

ITEM 7. 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.


                                       6

<PAGE>   7

ITEM 7. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND 
        RESULTS OF OPERATIONS - CONTINUED

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>
                                                        YEAR ENDED  DECEMBER 31,
                                                ------------------------------------
                                                  1998          1997          1996
                                                --------      --------      --------
<S>                                             <C>           <C>           <C>     
Riverside Avenue, Rialto, CA                    $ 85,545      $ 80,255      $ 67,599

Elden Avenue, Moreno Valley, CA                   82,084        73,557        72,988

Foothill Boulevard, La Verne, CA                 113,886       105,677       100,030

Baseline & Archibald, Rancho Cucamonga, CA        85,069        73,553        64,636

Elkhorn Boulevard, Sacramento, CA                 55,132        49,752        45,528

Haven Avenue, Rancho Cucamonga, CA               109,892       104,486       101,422
                                                --------      --------      --------
           Total                                $531,608      $487,280      $452,203
                                                ========      ========      ========
</TABLE>

The partnership receives rental revenues equal to 12 percent of gross sales from
the restaurants. The partnership earned rental revenue of $531,608 during the
year ended December 31, 1998, which represents an increase of $44,328 from 1997.
The increase in rental revenue was caused by an increase in sales at the
restaurants under lease.

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

                 PERCENTAGE OF TOTAL GENERAL & ADMIN. EXPENSE

<TABLE>
<CAPTION>
                                
                                           Year Ended December 31,
                                --------------------------------------------
                                  1998               1997              1996
                                -------             ------            ------
<S>                             <C>                 <C>               <C>   
Accounting fees                  35.29%              32.70%            36.02%
Distribution of
  information to
  limited partners               63.00               65.53             61.43
Other                             1.71                1.77              2.55
                                ------              ------            ------
                                100.00%             100.00%           100.00%
                                ======              ======            ======
</TABLE>


                                       7

<PAGE>   8

ITEM 7. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND
        RESULTS OF OPERATIONS - CONTINUED

General and administrative costs increased from 1997 to 1998 due to increased
costs for printing and distribution of documents. Depreciation expense was the
same in both 1997 and 1998.

Net income increased by $42,504 from 1997 to 1998 due to the increase in
revenues of $44,143 offset by the $1,639 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.




                                       8

<PAGE>   9

ITEM 8. FINANCIAL STATEMENTS

PART I. INFORMATION


              INDEX                                                  PAGE NUMBER
              -----                                                  -----------

Report of Independent Public Accountants                                  10

Balance Sheets at December 31, 1998 and 1997                              11

Statement of of Income for the years ended
  December 31, 1998, 1997 and 1996                                        12

Statement of Changes in Partners' Equity for
  the three years ended December 31, 1998                                 13

Statements of Cash Flows for the years ended
  December 31, 1998, 1997 and 1996                                        14

Notes to Financial Statements                                          15-19


                                       9

<PAGE>   10
                              ARTHUR ANDERSEN LLP

                    REPORT OF INDEPENDENT PUBLIC ACCOUNTANTS


To the Partners of
  Del Taco Restaurant Properties, I:

We have audited the accompanying balance sheets of Del Taco Restaurant 
Properties I (a California Limited Partnership) as of December 31, 1998 and 
1997, and the related statements of income, changes in partners' equity and cash
flows for each of the three years in the period ended December 31, 1998. These 
financial statements and the schedule referred to below are the responsibility 
of the Partnership's management. Our responsibility is to express an opinion on 
these financial statements.

We conducted our audits in accordance with generally accepted auditing 
standards. Those standards required that we plan and perform the audit to 
obtain reasonable assurance about whether the financial statements are free 
from material misstatement. An audit includes examining, on a test basis, 
evidence supporting the amounts and disclosures in the financial statements. An 
audit also includes assessing the accounting principles used and significant 
estimates made by management, as well as evaluating the overall financial 
statement presentation. We believe that our audits provide a reasonable basis 
for our opinion.

In our opinion, the financial statements referred to above present fairly, in 
all material respects, the financial position of Del Taco Restaurant Properties 
I as of December 31, 1998 and 1997, and the results of its operations and its 
cash flows for each of the three years in the period ended December 31, 1998, 
in conformity with generally accepted accounting principles.

Our audits were made for the purpose of forming an opinion on the basic 
financial statements taken as a whole. The schedule in the index of the 
financial statements is presented for purposes of complying with the Securities 
and Exchange Commission's rules and is not part of the basic financial 
statements. This schedule has been subjected to the auditing procedures applied 
in our audits of the basic financial statements and, in our opinion, fairly 
states in all material respects, the financial data required to be set forth 
therein in relation to the basic financial statements taken as a whole.


                                            /s/ ARTHUR ANDERSEN LLP
                                            -----------------------------------
                                                ARTHUR ANDERSEN LLP


Orange County, California
February 12, 1999

                                       10

<PAGE>   11

                        DEL TACO RESTAURANT PROPERTIES I

                                 BALANCE SHEETS

<TABLE>
<CAPTION>
                                                      DECEMBER 31,
                                               --------------------------
                                                  1998           1997
                                               ----------     -----------
<S>                                            <C>             <C>       
                                     ASSETS

CURRENT ASSETS:
  Cash                                         $  162,979      $  143,280
  Receivable from Del Taco, Inc. (Note 4)          47,833          42,601
  Deposits                                            797             600
                                               ----------      ----------
     Total current assets                         211,609         186,481
                                               ----------      ----------

PROPERTY AND EQUIPMENT, AT COST (NOTE 1):
  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              1,716,087       1,672,315
                                               ----------      ----------
                                                2,285,555       2,329,327
                                               ----------      ----------

                                               $2,497,164      $2,515,808
                                               ==========      ==========

                        LIABILITIES AND PARTNERS' EQUITY

CURRENT LIABILITIES:
  Payable to limited partners                  $   24,702      $   12,690
  Accounts payable                                  5,703           6,531
                                               ----------      ----------
     Total current liabilities                     30,405          19,221
                                               ----------      ----------

PARTNERS' EQUITY (NOTE 2):
  Limited partners                              2,201,963       2,231,634
  General Partner - Del Taco, Inc.                264,796         264,953
                                               ----------      ----------
                                                2,466,759       2,496,587
                                               ----------      ----------

                                               $2,497,164      $2,515,808
                                               ==========      ==========
</TABLE>

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

                                       11

<PAGE>   12

                        DEL TACO RESTAURANT PROPERTIES I

                              STATEMENTS OF INCOME

<TABLE>
<CAPTION>
                                         YEAR ENDED DECEMBER 31,
                                  ------------------------------------
                                    1998          1997          1996
                                  --------      --------      --------
<S>                               <C>           <C>           <C>     
REVENUES:
  Rent (Note 3)                   $531,608      $487,280      $452,203
  Interest                           3,425         3,359         2,265
  Other                              1,499         1,750           637
                                  --------      --------      --------
                                   536,532       492,389       455,105
                                  --------      --------      --------
EXPENSES:
  General and administrative        46,900        45,261        45,943
  Depreciation                      43,772        43,772        43,775
                                  --------      --------      --------
                                    90,672        89,033        89,718
                                  --------      --------      --------

Net income                        $445,860      $403,356      $365,387
                                  ========      ========      ========
Net income per limited
  partnership Unit (Note 1)       $  50.44      $  45.63      $  41.34
                                  ========      ========      ========
</TABLE>


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


                                       12

<PAGE>   13

                        DEL TACO RESTAURANT PROPERTIES I

                    STATEMENT OF CHANGES IN PARTNERS' EQUITY

                       THREE YEARS ENDED DECEMBER 31, 1998

<TABLE>
<CAPTION>
                                          Limited Partners
                                    ----------------------------          General
                                      Units             Amount            Partner            Total
                                    -----------      -----------       -----------       -----------
<S>                                  <C>             <C>               <C>               <C>        
Balance, December 31, 1995                8,751      $ 2,293,755       $   265,580       $ 2,559,335
   Net income                                --          361,733             3,654           365,387
   Cash distributions (Note 6)               --         (391,909)           (3,959)         (395,868)
                                    -----------      -----------       -----------       -----------
Balance, December 31, 1996                8,751        2,263,579           265,275         2,528,854
   Net income                                --          399,322             4,034           403,356
   Cash distributions (Note 6)               --         (431,267)           (4,356)         (435,623)
                                    -----------      -----------       -----------       -----------
Balance, December 31, 1997                8,751        2,231,634           264,953         2,496,587
   Net income                                --          441,401             4,459           445,860
   Cash distributions (Note 6)               --         (471,072)           (4,616)         (475,688)
                                    -----------      -----------       -----------       -----------
Balance, December 31, 1998                8,751      $ 2,201,963       $   264,796       $ 2,466,759
                                    ===========      ===========       ===========       ===========
</TABLE>

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


                                       13

<PAGE>   14

                        DEL TACO RESTAURANT PROPERTIES I

                            STATEMENTS OF CASH FLOWS

<TABLE>
<CAPTION>
                                                               YEAR ENDED DECEMBER 31,
                                                     -----------------------------------------
                                                        1998           1997             1996
                                                     ----------      ---------       ---------
<S>                                                  <C>             <C>             <C>      
CASH FLOWS FROM OPERATING ACTIVITIES:

Net income                                           $ 445,860       $ 403,356       $ 365,387
Adjustments to reconcile net income to
 net cash provided by operating
 activities:
    Depreciation                                        43,772          43,772          43,775
    Increase in payable to limited partners             12,012           1,371           2,421
    Increase in receivable from General Partner         (5,232)         (4,123)         (1,824)
    (Decrease) increase in accounts payable               (828)          3,531           2,093
    Increase in deposits                                  (197)             --              --
                                                     ---------       ---------       ---------

     Net cash provided by operating activities         495,387         447,907         411,852


CASH FLOWS FROM FINANCING ACTIVITIES:

Cash distributions to partners                        (475,688)       (435,623)       (395,868)
                                                     ---------       ---------       ---------

NET INCREASE IN CASH                                    19,699          12,284          15,984

BEGINNING CASH BALANCE                                 143,280         130,996         115,012
                                                     ---------       ---------       ---------

ENDING CASH BALANCE                                  $ 162,979       $ 143,280       $ 130,996
                                                     =========       =========       =========
</TABLE>


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

                                       14

<PAGE>   15

                        DEL TACO RESTAURANT PROPERTIES I

                          NOTES TO FINANCIAL STATEMENTS

                                DECEMBER 31, 1998


NOTE 1 - ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

THE PARTNERSHIP: Del Taco Restaurant Properties I (a California limited
partnership) was formed on November 30, 1982, for the purpose of acquiring real
property in California for construction of six Mexican-American restaurants to
be leased under long-term agreements to Del Taco, Inc. (General Partner for
operation under the Del Taco trade name).

BASIS OF ACCOUNTING: The partnership utilizes the accrual method of accounting
for transactions relating to the business of the partnership. Distributions are
made to the general and limited partners in accordance with the provisions of
the partnership agreement (see Note 2).

PROPERTY AND EQUIPMENT: Property and equipment is stated at cost. Depreciation
is computed using the straight-line method over estimated useful lives which are
20 years for land improvements, 35 years for buildings and improvements, and 10
years for machinery and equipment.

In fiscal 1996, the partnership adopted Statement of Financial Accounting
Standards (SFAS) No. 121, "Accounting for the Impairment of Long Lived Assets
and for Long Lived Assets to be Disposed of." SFAS 121 requires that long-lived
assets be reviewed for impairment whenever events or changes in circumstances
indicate that the carrying value of the asset may not be recoverable. In
evaluating long-lived assets held for use, an impairment loss is recognized if
the sum of the expected future cash flows (undiscounted and without interest
charges) is less than the carrying value of the asset. Once a determination has
been made that an impairment loss should be recognized for long-lived assets,
various assumptions and estimates are used to determine fair value including,
among others, estimated costs of construction and development, recent sales of
comparable properties and the opinions of fair value prepared by independent
real estate appraisers. Long-lived assets to be disposed of are reported at the
lower of carrying amount or fair value less cost to sell.

The adoption of SFAS No. 121 did not have a material effect on the partnership's
financial statements.


                                       15

<PAGE>   16

DEL TACO RESTAURANT PROPERTIES I
NOTES TO FINANCIAL STATEMENTS - CONTINUED
DECEMBER 31, 1998


NOTE 1 - ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES -
         (CONTINUED)

INCOME TAXES: No provision has been made for federal or state income taxes on
partnership net income, since the partnership is not subject to income tax.
Partnership income is includable in the taxable income of the individual
partners as required under applicable income tax laws. Certain items, primarily
related to depreciation methods, are accounted for differently for income tax
reporting purposes (see Note 5).

NET INCOME PER LIMITED PARTNERSHIP UNIT: Net income per limited partnership unit
is based upon the weighted average number of units outstanding during the period
which amounted to 8,751 for all years presented.

USE OF ESTIMATES: The preparation of the financial statements in conformity with
generally accepted accounting principles requires management to make certain
estimates and assumptions that affect the reported amounts of assets and
liabilities at the date of the financial statements and the reported amounts of
revenues and expenses during the reporting period. Actual results could differ
from those estimates.

NOTE 2 - PARTNERS' EQUITY

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.

In 1986, the General Partner contributed additional capital of $280,000 to the
partnership in order to provide funds necessary to complete the sixth and final
restaurant.


                                       16

<PAGE>   17

DEL TACO RESTAURANT PROPERTIES I
NOTES TO FINANCIAL STATEMENTS - CONTINUED
DECEMBER 31, 1998


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. The
partnership had a total of six properties leased as of December 31, 1998, 1997
and 1996, two of which have been subleased to Del Taco franchisees (one of which
is affiliated with Del Taco, Inc.)

The five restaurants operated by or affiliated with Del Taco, for which the
partnership is the lessor, had combined, unaudited sales of $3,721,166,
$3,447,723, and $3,229,726 and unaudited net income of $207,430, $149,010, and
$77,336 for the years ended December 31, 1998, 1997 and 1996, respectively. Net
income by restaurant includes charges for general and administrative expenses
incurred in connection with supervision of restaurant operations and interest
expense. The one restaurant operated by a Del Taco franchisee, for which the
partnership is the lessor, had unaudited sales of $708,908, $612,943 and
$538,632 for the years ended December 31, 1998, 1997 and 1996, respectively.

The Elkhorn Boulevard restaurant in Sacramento, California had unaudited net
losses of $13,373, $13,302, and $18,566 for the years ended December 31, 1998,
1997 and 1996, respectively.

NOTE 4 - RELATED PARTIES

The receivable from Del Taco consists of rent accrued for the month of December
1998. The rent receivable was collected on January 17, 1999.

The General Partner received $4,616 in distributions relating to its one percent
interest in the partnership for the year ended December 31, 1998.

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.


                                       17

<PAGE>   18

DEL TACO RESTAURANT PROPERTIES I
NOTES TO FINANCIAL STATEMENTS - CONTINUED
DECEMBER 31, 1998


NOTE 5 - INCOME TAXES

A reconciliation of financial statement net income to taxable income for each of
the periods is as follows:

<TABLE>
<CAPTION>
                                           1998            1997            1996
                                         ---------       ---------       ---------
<S>                                      <C>             <C>             <C>      
Net income per financial statements      $ 445,860       $ 403,356       $ 365,387

Excess book depreciation                      (670)         (2,265)         (4,780)
                                         ---------       ---------       ---------
Taxable income                           $ 445,190       $ 401,091       $ 360,607
                                         =========       =========       =========
</TABLE>

A reconciliation of partnership equity per the financial statements to net worth
for tax purposes as of December 31, 1998, is as follows:

Partners' equity per financial
  statements                                              $2,466,759

Issue costs of limited partnership
  units capitalized for tax purposes                         637,325

Excess tax depreciation                                     (138,003)

Other                                                            235
                                                          ----------
Net worth for tax purposes                                $2,966,316
                                                          ==========


                                       18

<PAGE>   19

DEL TACO RESTAURANT PROPERTIES I
NOTES TO FINANCIAL STATEMENTS - CONTINUED
DECEMBER 31, 1998


NOTE 6 - CASH DISTRIBUTIONS TO LIMITED PARTNERS

Cash distributions paid to limited partners for the three years ended December
31, 1998 were as follows:

<TABLE>
<CAPTION>
                                                Cash                Weighted         Number of Units
                                         Distribution per        Average Number       Outstanding at
                                        Limited Partnership         of Units            the End of
Quarter Ended                                   Unit              Outstanding            Quarter
-------------                           -------------------     ---------------      ---------------
<S>                                            <C>                    <C>                  <C>  
December 31, 1995                              $11.46                 8,751                8,751
March 31, 1996                                  10.68                 8,751                8,751
June 30, 1996                                   10.21                 8,751                8,751
September 30, 1996                              12.47                 8,751                8,751
                                               ------
  Total paid in 1996                           $44.82
                                               ======

December 31, 1996                              $12.63                 8,751                8,751
March 31, 1997                                  11.59                 8,751                8,751
June 30, 1997                                   11.19                 8,751                8,751
September 30, 1997                              13.87                 8,751                8,751
                                               ------
  Total paid in 1997                           $49.28
                                               ======

December 31, 1997                              $14.21                 8,751                8,751
March 31, 1998                                  10.82                 8,751                8,751
June 30, 1998                                   13.34                 8,751                8,751
September 30, 1998                              15.47                 8,751                8,751
                                               ------
  Total paid in 1998                           $53.84
                                               ======
</TABLE>

Cash distributions per limited partnership unit were calculated based upon the
weighted average number of units outstanding for each quarter and were paid from
operations. Cash distributions for the quarter ended December 31, 1998 amounted
to $15.08 per limited partnership unit and were paid January 31, 1999.


                                       19

<PAGE>   20

                                    PART III


ITEM 9. DISAGREEMENTS ON ACCOUNTING AND FINANCIAL DISCLOSURE

None

ITEM 10. DIRECTORS AND EXECUTIVE OFFICERS OF THE PARTNERSHIP'S GENERAL PARTNER

(a) & (b) The executive officers and directors of the General Partner and their
ages are set forth below:

<TABLE>
<CAPTION>

Name                                Title                                                         Age
----                                -----                                                         ---
<S>                          <C>                                                                  <C>
Kevin K. Moriarty            Director, Chairman and Chief Executive Officer                       52

C. Ronald Petty              President                                                            54

Paul W. Hitzelberger         Executive Vice President, Brand Strategy and  Franchise
                             Relations/Development                                                54

Robert Terrano               Executive Vice President and  Chief Financial Officer                43

James D. Stoops              Executive Vice President, Operations                                 46

Janet D. Simmons             Senior Vice President, Purchasing                                    42

Michael L. Annis             Vice President, Secretary and General Counsel                        52

C. Douglas Mitchell          Vice President and Corporate Controller                              48

Annette Shehan               Vice President, Marketing                                            37

Shirlene Lopez               Vice President, Corporate Development                                34
</TABLE>


The above referenced executive officers and directors of the General Partner
will hold office until the annual meeting of its shareholders and directors,
which is scheduled for the later part of 1999.

(c) None

(d) No family relationship exists between any such director or executive officer
    of the General Partner.

(e) The following is an account of the business experience during the past five
    years of each such director and executive officer:


                                       20

<PAGE>   21

KEVIN K. MORIARTY, Director, Chairman and Chief Executive Officer of Del Taco,
Inc. Mr. Moriarty began his career with Burger King Corporation in 1974 in
Operations Unit Management. In 1983, he was promoted to Area Manager in New
York, and was subsequently promoted to the Regional Vice President, Chicago
Region in 1985. In 1988, he became Executive Vice President and General Manager
of the North Central Division. Mr. Moriarty served in that position until 1990
when he joined Del Taco, Inc. as President and Chief Executive Officer on July
31, 1990. Mr. Moriarty has served as a Director of the General Partner since
1990.

C. RONALD PETTY, President of Del Taco, Inc. Mr. Petty began his career in the
restaurant business in 1973 with McDonald's Corporation. He was employed by
McDonald's in a real estate capacity until 1978. For the next 12 years, Mr.
Petty was in various officer positions with Burger King. These positions
included Vice President of Real Estate, Sr. Vice President of Development,
Region Vice President, Sr. Vice President European Operations, President of
International and President of U.S. Mr. Petty served as President of Miami Subs
from 1990-1992; President and CEO of Denny's 1993-1996; President and CEO of
Peter Piper Pizza 1996-1998; President of Del Taco December 1998-present.

PAUL W. HITZELBERGER, Executive Vice President, Brand Strategy and Franchise
Relations/Development of Del Taco, Inc. He was appointed to his current position
in December 1995. Previously, Mr. Hitzelberger was Executive Vice President,
Marketing from February 1991 and Vice President, Marketing from 1989 to 1991.
Mr. Hitzelberger has responsibility for franchise development, relations and
training and will oversee public relations and training for the corporation.
From September 1988 through September 1989, Mr. Hitzelberger was Chief Executive
Officer of Environmental Marketing Group. Prior to that, Mr. Hitzelberger was a
Vice President of Del Taco, Inc. Prior to joining Del Taco, Inc., he served as
Vice President - Marketing at the department store division of Lucky Stores,
Inc., a major supermarket retailer. Prior to his position with Lucky, Mr.
Hitzelberger held various positions in marketing and retailing at Wallpapers to
Go, Inc., a division of General Mills, Inc., and Coast to Coast Stores, Inc. a
subsidiary of Household Merchandising, Inc. Mr. Hitzelberger received a Master
of Business Administration degree from Loyola University in Chicago, Illinois.

ROBERT J. TERRANO, Executive Vice President and Chief Financial Officer of Del
Taco, Inc. From May 1994 to April 1995, Mr. Terrano served as Chief Financial
Officer for Denny's, Inc. in Spartanburg, S.C. From August 1983 to May 1994, he
served with Burger King Corporation, Miami Florida, in a variety of positions,
most recently as Division Controller. Mr. Terrano joined Del Taco, Inc. in April
1995.

JAMES D. STOOPS, Executive Vice President, Operations of Del Taco, Inc. From
1968 to 1991, Mr. Stoops served in a wide variety of Operations positions with
Burger King Corporation with increasing levels of responsibility. In 1985, Mr.
Stoops was appointed Region Vice President/General Manager for the New York
region and served in that position until October of 1990. In January of 1991, he
joined Del Taco, Inc. in his current post.

JANET D. SIMMONS, Senior Vice President, Purchasing of Del Taco, Inc. From 1979
to 1986, Ms. Simmons was with Denny's Incorporated. She served in the Research
and Development department in a variety of positions until 1982 when she was
promoted to the position of Purchasing Agent. Ms. Simmons was hired in 1986 as
Manager of Contract Purchasing with Carl Karcher Enterprises, a post she held
until March 1990 when she became Vice President, Purchasing for Del Taco, Inc.
Ms. Simmons has a Bachelor of Science degree in Foods and Nutrition from Cal
State Polytechnic University in Pomona, California.


                                       21

<PAGE>   22

MICHAEL L. ANNIS, Vice President, Secretary and General Counsel of Del Taco,
Inc. From 1981 to 1986 Mr. Annis served as Regional Real Estate Manager and
Director of Real Estate Services with Taco Bell, Inc. In 1986 he served as
Regional General Manager with Quaker State Minit Lube. In January of 1987 Mr.
Annis joined Red Robin International, Inc. as General Counsel and was
subsequently promoted to Vice President/Secretary and later Vice President Real
Estate Development/Secretary and General Counsel, the position he held until
joining Del Taco, Inc. in December of 1993. Mr. Annis received his J.D. Degree
from Whittier College.

C. DOUGLAS MITCHELL, Vice President and Corporate Controller. Mr. Mitchell
joined Del Taco, Inc. in August of 1994 as Controller and was promoted to his
current position in January 1996. From 1990 to 1994, Mr. Mitchell was a Senior
Audit Manager with Coopers & Lybrand. Prior to 1990, Mr. Mitchell held various
positions in finance and accounting with the Geneva Companies (a subsidiary of
Chemical Bank), Zaremba Corporation (a real estate developer) and The Dexter
Corporation (an international manufacturer of specialty materials). Mr. Mitchell
has a Bachelor of Science degree with a major in accounting from the University
of Southern California.

ANNETTE D. SHEHAN, Vice President, Marketing of Del Taco, Inc. From 1984-1994
Ms. Shehan was with Jack In The Box Restaurants where she held various positions
in Research & Development, Product Marketing and Advertising/Media. Ms. Shehan
was hired in 1994 as Manager of Broadcast Advertising with HomeBase, Inc. a home
improvement retailer, a post she held until 1998 when she became Vice President,
Marketing for Del Taco, Inc. Ms. Shehan has a Bachelor of Science Degree in Food
Science from California Polytechnic University, San Luis Obispo and a Master of
Business Administration from San Diego State University.

SHIRLENE LOPEZ, Vice President, Corporate Development & Design of Del Taco, Inc.
Ms. Lopez began her career with Del Taco in 1978 as an hourly employee and
advanced through the ranks to General Manager in 1984. Ms. Lopez was promoted to
the corporate office in 1989 as Human Resource Manager in support of the field
and office manager. In 1994, she was promoted to Executive Project Manager of
reporting to the CEO and in 1996, to Director of Corporate Development in charge
of all interior image and design. Ms. Lopez has held her current position since
August 1997.

ITEM 11. MANAGEMENT REMUNERATION AND TRANSACTIONS

The partnership has no executive officers or directors and pays no direct
remuneration to any executive officer or director of its General Partner. The
partnership has not issued any options or stock appreciation rights to any
executive officer or director of its General Partner, nor does the partnership
propose to pay any annuity, pension or retirement benefits to any executive
officer or director of its General Partner. The partnership has no plan, nor
does the partnership presently propose a plan, which will result in any
remuneration being paid to any executive officer or director of the General
Partner upon termination of employment.


                                       22

<PAGE>   23

ITEM 12. SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

(a) No person of record currently owns more than five percent of limited
    partnership units of the partnership, nor was any person known of by the
    partnership to own of record and beneficially, or beneficially only, more
    than five percent of such securities.

(b) Neither Del Taco, Inc., nor any executive officer or director of Del Taco,
    Inc. owns any limited partnership units of the partnership.

(c) The partnership knows of no contractual arrangements, the operation or the
    terms of which may at a subsequent date result in a change in control of the
    partnership , except for provisions in the partnership agreement providing
    for removal of the General Partner by holders of a majority of the limited
    partnership units and if a material event of default occurs under the
    financing agreements of the General Partner.

ITEM 13. CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS

(a) No transactions have occurred between the partnership and any executive
    officer or director of its General Partner.

        During 1998, the following transactions occurred between the partnership
        and the General Partner pursuant to the terms of the partnership
        agreement.

    (1) The General Partner earned $4,459 as its one percent share of the net 
        income of the partnership.

    (2) The General Partner received $4,616 in distributions relating to its
        one percent interest in the partnership.

(b) During 1998, the partnership had no business relationships with any entity
    of a type required to be reported under this item.

(c) Neither the General Partner, any director or officer of the General Partner
    or any associate of any such person, was indebted to the partnership at any
    time during 1998 for any amount in excess of $60,000.

(d) Not applicable.


                                       23

<PAGE>   24

                                     PART IV


ITEM 14(a)(1) AND (2). EXHIBITS, FINANCIAL STATEMENTS SCHEDULES, AND REPORTS ON
                       FORM 8-K


    Financial statement schedules:

    Schedule III - Real Estate and Accumulated Depreciation

    Financial statement schedules other than those referred to above have been
    omitted because they are not applicable or not required.


(b) No reports on Form 8-K were filed during the last quarter of 1998.

(c) Exhibits required by Item 601 of Regulation S-K:

    1. Incorporated herein by reference, Restated Certificate and Agreement of
       Limited Partnership of Del Taco Restaurant Properties I filed as Exhibit
       3.01 to Partnership's Registration Statement on Form S-11 as filed with
       the Securities and Exchange Commission on December 17, 1982.

    2. Incorporated herein by reference, Amendment to Restated Certificate and
       Agreement of Limited Partnership of Del Taco Restaurant Properties I.

    3. Incorporated herein by reference, Form of Standard Lease to be entered
       into by partnership and Del Taco, Inc., as lessee, filed as Exhibit 10.02
       to Partnership's Registration Statement on Form S-11 as filed with the
       Securities and Exchange Commission on December 17, 1982.


                                       24

<PAGE>   25
                 DEL TACO RESTAURANT PROPERTIES I - SCHEDULE III

                    REAL ESTATE AND ACCUMULATED DEPRECIATION
                                DECEMBER 31, 1998
<TABLE>
<CAPTION>
                                                                                   Cost capitalized   Gross amount at
                                                         Initial cost                subsequent to    which carried at
           Description                                    to company                  acquisition     close of period
           -----------                            ---------------------------------------------------------------------
                                                     Land           Building &                        Land, buildings
                                                    & land           Improve-           Carrying      & improvements    Accumulated
(All Restaurants)                    Encumbrances  improvements       ments              costs             Total        depreciation
------------------------------------------------------------------------------------------------------------------------------------
<S>                                  <C>           <C>             <C>                  <C>           <C>               <C>
Rialto, CA                                $--      $  274,837      $  150,310             $--           $  425,147      $   86,059
Moreno Valley, CA                          --         353,557         193,362              --              546,919         110,708
La Verne, CA                               --         452,423         247,433              --              699,856         141,665
Rancho Cucamonga, CA                       --         293,817         160,690              --              454,507          92,001
Sacramento, CA                             --         260,516         142,478              --              402,994          81,575
Rancho Cucamonga, CA                       --         217,332         118,861              --              336,193          68,053
                                          -----------------------------------------------------------------------------------------
                                          $--      $1,852,482      $1,013,134             $--           $2,865,616      $  580,061
                                          =========================================================================================
</TABLE>

<TABLE>
<CAPTION>

                                                                Life on which
                                                            depreciation in latest
                                    Date of       Date         income statement
                                  construction  acquired         is computed
                                  ------------  --------    ----------------------
<S>                               <C>           <C>         <C>
Rialto, CA                           1984         1984         20 (LI), 35 (BI)
Moreno Valley, CA                    1985         1985         20 (LI), 35 (BI)
La Verne, CA                         1985         1985         20 (LI), 35 (BI)
Rancho Cucamonga, CA                 1985         1985         20 (LI), 35 (BI)
Sacramento, CA                       1985         1985         20 (LI), 35 (BI)
Rancho Cucamonga, CA                 1985         1985         20 (LI), 35 (BI)
</TABLE>


<TABLE>
<CAPTION>
                                                                   Accumulated
                                                Restaurants        Depreciation
                                                -----------        ------------
<S>                                             <C>                 <C>      
Balances at December 31, 1995:                  $ 2,865,616         $ 448,745
   Acquisitions                                          --            43,772
   Sales                                                 --                --
                                                -----------         ---------
Balances at December 31, 1996:                    2,865,616           492,517
   Acquisitions                                          --            43,772
   Sales                                                 --                --
                                                -----------         ---------
Balances at December 31, 1997:                    2,865,616           536,289
   Acquisitions                                          --            43,772
   Sales                                                 --                --
                                                -----------         ---------
Balances at December 31, 1998:                  $ 2,865,616         $ 580,061
                                                ===========         =========
</TABLE>


                                       25
<PAGE>   26

                                   SIGNATURES


Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange
Act of 1934, the partnership 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

                                            Del Taco, Inc.
                                            General Partner



Date March 07, 1999                         Kevin K. Moriarty
     --------------                         -----------------
                                            Kevin K. Moriarty
                                            Director, Chairman and Chief
                                            Executive Officer



Date March 07, 1999                         Michael L. Annis
     --------------                         ----------------
                                            Michael L. Annis
                                            Vice President, Secretary and
                                            General Counsel



Date March 07, 1999                         Robert J. Terrano
     --------------                         -----------------
                                            Robert J. Terrano
                                            Executive Vice President and
                                            Chief Financial Officer



Date March 07, 1999                         C. Douglas Mitchell
     --------------                         -------------------
                                            C. Douglas Mitchell
                                            Vice President and Corporate 
                                            Controller


                                       26


<PAGE>   27

                                 EXHIBIT INDEX

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

27.1                Financial Data Schedule