SEC File Number:

1-14023

CUSIP Number:

22002T108

(Check one):

o Form 10-K

o Form 20-F

x Form 11-K

oForm 10-Q

o Form 10-D

o Form N-SAR

o Form N-CSR

For Period Ended:

o Transition Report on Form 10-K

o Transition Report on Form 20-F

o Transition Report on Form 11-K

o Transition Report on Form 10-Q

o Transition Report on Form N-SAR

For the Transition Period Ended:

PART I — REGISTRANT INFORMATION

Corporate Office Properties Trust

Full Name of Registrant

Corporate Office Properties, L.P. Employee Retirement Savings Plan

Full Name of Plan:

N/A

Former Name if Applicable

8815 Centre Park Drive, Suite 400

Address of Principal Executive Office (Street and Number)

Columbia, Maryland 21045

City, State and Zip Code

PART II — RULES 12b-25(b) AND (c)

If the subject report could not be filed without unreasonable effort or expense and the registrant seeks relief pursuant to
Rule 12b-25(b), the following should be completed. (Check box if appropriate)

(a)

The reason described in reasonable detail in Part III of this form could not be eliminated without unreasonable effort or expense;

x

(b)

The subject annual report, semi-annual report, transition report on Form 10-K, Form 20-F, Form 11-K, Form N-SAR or Form N-CSR, or portion thereof, will be filed on or before the fifteenth calendar day following the prescribed due date; or the subject quarterly report or transition report on Form 10-Q or subject distribution report on Form 10-D, or portion thereof, will be filed on or before the fifth calendar day following the prescribed due date; and

(c)

The accountant’s statement or other exhibit required by Rule 12b-25(c) has been attached if applicable.

(1)

Name and telephone number of person to contact in regard to this notification

Karen M. Singer, Vice President and General Counsel

(410)

992-7293

(Name)

(Area Code)

(Telephone Number)

(2)

Have all other periodic reports required under Section 13 or 15(d) of the Securities Exchange Act of 1934 or Section 30 of the Investment Company Act of 1940 during the preceding 12 months or for such shorter period that the registrant was required to file such report(s) been filed ? If answer is no, identify report(s).

x Yes    o No

(3)

Is it anticipated that any significant change in results of operations from the corresponding period for the last fiscal year will be reflected by the earnings statements to be included in the subject report or portion thereof?

o Yes    x No

If so: attach an explanation of the anticipated change, both narratively and quantitatively, and, if appropriate, state the reasons why a reasonable estimate of the results cannot be made.

Corporate Office Properties Trust

(Name of Registrant as Specified in Charter)

Date

June 30, 2006

By

/s/ Randall M. Griffin

Name:

Randall M. Griffin

Title

President and Chief Executive Officer

ATTENTION

Intentional misstatements or omissions of fact constitute Federal Criminal Violations (See 18 U.S.C. 1001).

SEC 1344(05-06)

Persons who are to respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB control number.