SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
Schedule 13G
Under the Securities Exchange Act of 1934
(Amendment No. )*
ACADIA PHARMACEUTICALS INC
_______________________________________
(Name of Issuer)
Common Stock
_______________________________________
(Title of Class of Securities)
004225108
_______________________________________
(CUSIP Number)
Check the following box if a fee is being paid with this statement.
(A fee is not required only if the filing person: (1) has a
previous statement on file reporting beneficial ownership of more
than five percent of the class of securities described in Item 1;
and (2) has filed no amendment subsequent thereto reporting
beneficial ownership of five percent or less of such class.) (See
Rule 13d-7.)
*The remainder of this cover page shall be filled out for a
reporting person's initial filing on this form with respect to the
subject class of securities, and for any subsequent amendment
containing information which would alter the disclosures provided
in a prior cover page.
The information required in the remainder of this cover page shall
not be deemed to be "filed" for the purpose of Section 18 of the
Securities Exchange Act of 1934 ("Act") or otherwise subject to the
liabilities of that section of the Act but shall be subject to all
other provisions of the Act (however, see the Notes).
(Continued on following page(s))
Page 1 of 8 Pages
CUSIP NO. 004225108 13G Page 2 of 8 Pages
1 Name of Reporting Person
S.S. or I.R.S. Identification No. of Above Person
T. ROWE PRICE ASSOCIATES, INC.
52-0556948
2 Check the Appropriate Box if a Member of a Group*
(a) ____
NOT APPLICABLE (b) ____
3 SEC Use Only
______________________________
4 Citizenship or Place of Organization
MARYLAND
Number of 5 Sole Voting Power
**
Shares 36,000
Beneficially 6 Shared Voting Power
**
Owned By Each NONE
Reporting 7 Sole Dispositive Power
**
Person 1,482,621
With 8 Shared Dispositive Power
NONE
9 Aggregate Amount Beneficially Owned by Each Reporting Person
1,482,621
10 Check Box if the Aggregate Amount in Row (9) Excludes Certain
Shares*
NOT APPLICABLE
11 Percent of Class Represented by Amount in Row 9
6.2%
12 Type of Reporting Person*
IA
*SEE INSTRUCTION BEFORE FILLING OUT!
**Any shares reported in Items 5 and 6 are also
reported in Item 7.
CUSIP NO. 004225108 13G Page 3 of 8 Pages
1 Name of Reporting Person
S.S. or I.R.S. Identification No. of Above Person
T. ROWE PRICE NEW HORIZONS FUND, INC.
52-0791372
2 Check the Appropriate Box if a Member of a Group*
(a) ____
NOT APPLICABLE (b) ____
3 SEC Use Only
______________________________
4 Citizenship or Place of Organization
MARYLAND
Number of 5 Sole Voting Power
**
Shares 1,442,221
Beneficially 6 Shared Voting Power
**
Owned By Each NONE
Reporting 7 Sole Dispositive Power
**
Person NONE
With 8 Shared Dispositive Power
NONE
9 Aggregate Amount Beneficially Owned by Each Reporting Person
1,442,221
10 Check Box if the Aggregate Amount in Row (9) Excludes Certain
Shares*
NOT APPLICABLE
11 Percent of Class Represented by Amount in Row 9
6.1%
12 Type of Reporting Person*
IV
*SEE INSTRUCTION BEFORE FILLING OUT!
**The aggregate amount reported on this page is also included in
the aggregate amount reported by T. Rowe Price Associates, Inc. on
page 2 of this Schedule 13G.
SCHEDULE 13G
PAGE 4 OF 8
Item 1(a) Name of Issuer:
Reference is made to page 1 of this Schedule 13G
Item 1(b) Address of Issuer's Principal Executive Offices:
3911 Sorrento Valley Blvd, San Diego, CA 92121
Item 2(a) Name of Person(s) Filing:
(1) T. Rowe Price Associates, Inc. ("Price Associates")
(2) T. Rowe Price New Horizons Fund, Inc.
X Attached as Exhibit A is a copy of an agreement between
the Persons Filing (as specified hereinabove) that this
Schedule 13G is being filed on behalf of each of them.
Item 2(b) Address of Principal Business Office:
100 E. Pratt Street, Baltimore, Maryland 21202
Item 2(c) Citizenship or Place of Organization:
(1) Maryland
(2) Maryland
Item 2(d) Title of Class of Securities:
Reference is made to page 1 of this Schedule 13G
Item 2(e) CUSIP Number: 004225108
Item 3 The person filing this Schedule 13G is an:
X Investment Adviser registered under Section 203 of the
Investment Advisers Act of 1940
X Investment Company registered under Section 8 of the
Investment Company Act of 1940
Item 4 Reference is made to Items 5-11 on page 2 of this
Schedule 13G.
CUSIP 004225108
PAGE 5 OF 8
Item 4 Ownership Deemed
Outstanding
And
Beneficially
Units Owned Directly
Deemed Subject to
Beneficially Warrants &
Owned Conversion
Directly Privileges Total
____________ ______________ _______
(1) WITH RESPECT TO
PRICE ASSOCIATES
(includes shares
reported in
(2) below):
(a)Amount
Beneficially
Owned . . . . . . 1,352,621 130,000 1,482,621
(b)Percent of
Class . . . . . . . . . . . . . . . . . . . . . . 6.2%
(c)Number of
units as
to which
such
person has: . . .
(i) *sole power
to vote or
to direct
the vote . . . 36,000 -0- 36,000
(ii) *shared power
to vote or
to direct
the vote . . . -0- -0- -0-
(iii) *sole power
to dispose or
to direct the
disposition
of . . . . . 1,352,621 130,000 1,482,621
(iv) *shared power
to dispose or
to direct the
disposition
of . . . . . -0- -0- -0-
Page 6 of 8
Cusip No. 004225108
(2) WITH RESPECT TO
T. Rowe Price New Horizons Fund, Inc.
(if this is a
joint filing):
(a)Amount
Beneficially
Owned . . . . . . 1,312,221 130,000 1,442,221
(b)Percent of
Class . . . . . . . . . . . . . . . . . . . . . . 6.1%
(c)Number of
units as
to which
such person
has . . . . . . .
(i) *sole power to
vote or to
direct the
vote . . . . 1,312,221 130,000 1,442,221
(ii) *shared power
to vote or
to direct
the vote . . . -0- -0- -0-
(iii) *sole power
to dispose or
to direct the
disposition
of . . . . . -0- -0- -0-
(iv) *shared power
to dispose or
to direct the
disposition
of . . . . . -0- -0- -0-
*Units reported in subcategories (i) and (ii) are also included in
subcategory (iii) or (iv).
SCHEDULE 13G
PAGE 7 OF 8
Item 5 Ownership of Five Percent or Less of a Class.
X Not Applicable.
_____ This statement is being filed to report the fact that, as
of the date of this report, the reporting person(s) has
(have) ceased to be the beneficial owner of more than five
percent of the class of securities.
Item 6 Ownership of More than Five Percent on Behalf of Another
Person
(1) Price Associates does not serve as custodian of the
assets of any of its clients; accordingly, in each
instance only the client or the client's custodian or
trustee bank has the right to receive dividends paid
with respect to, and proceeds from the sale of, such
securities.
The ultimate power to direct the receipt of dividends
paid with respect to, and the proceeds from the sale
of, such securities, is vested in the individual and
institutional clients which Price Associates serves as
investment adviser. Any and all discretionary
authority which has been delegated to Price Associates
may be revoked in whole or in part at any time.
Except as may be indicated if this is a joint filing
with one of the registered investment companies
sponsored by Price Associates which it also serves as
investment adviser ("T. Rowe Price Funds"), not more
than 5% of the class of such securities is owned by any
one client subject to the investment advice of Price
Associates.
(2) With respect to securities owned by any one of the T.
Rowe Price Funds, only State Street Bank and Trust
Company, as custodian for each of such Funds, has the
right to receive dividends paid with respect to, and
proceeds from the sale of, such securities. No other
person is known to have such right, except that the
shareholders of each such Fund participate
proportionately in any dividends and distributions so
paid.
Item 7 Identification and Classification of the Subsidiary Which
Acquired the Security Being Reported on By the Parent
Holding Company.
Not Applicable.
Item 8 Identification and Classification of Members of the Group.
Not Applicable.
SCHEDULE 13G
PAGE 8 OF 8
Item 9 Notice of Dissolution of Group.
Not Applicable.
Item 10 Certification.
By signing below I (we) certify that, to the best of my
(our) knowledge and belief, the securities referred to
above were acquired in the ordinary course of business
and were not acquired for the purpose of and do not have
the effect of changing or influencing the control of the
issuer of such securities and were not acquired in
connection with or as a participant in any transaction
having such purpose or effect. T. Rowe Price
Associates, Inc. hereby declares and affirms that the
filing of Schedule 13G shall not be construed as an
admission that Price Associates is the beneficial owner
of the securities referred to, which beneficial
ownership is expressly denied.
Signature.
After reasonable inquiry and to the best of my (our)
knowledge and belief, I (we) certify that the
information set forth in this statement is true,
complete and correct.
Dated: February 14, 2006 Dated: February 14, 2006
T. ROWE PRICE NEW HORIZONS T. ROWE PRICE ASSOCIATES, INC.
FUND, INC.
By: /s/ Joseph A. Carrier By: /s/ Henry H. Hopkins
Joseph A. Carrier, Henry H. Hopkins,
Treasurer Vice President
Note: This Schedule 13G, including all exhibits, must be filed
with the Securities and Exchange Commission, and a copy
hereof must be sent to the issuer by registered or
certified mail not later than February 14th following
the calendar year covered by the statement or within the
time specified in Rule 13d-1(b)(2), if applicable.
12/31/2005
EXHIBIT A
AGREEMENT
JOINT FILING OF SCHEDULE 13G
T. Rowe Price Associates, Inc. (an investment adviser
registered under the Investment Advisers Act of 1940), T. Rowe
Price New Horizons Fund, Inc., a Maryland corporation, hereby agree
to file jointly the statement on Schedule 13G to which this
Agreement is attached, and any amendments thereto which may be
deemed necessary, pursuant to Regulation 13D-G under the Securities
Exchange Act of 1934.
It is understood and agreed that each of the parties hereto
is responsible for the timely filing of such statement and any
amendments thereto, and for the completeness and accuracy of the
information concerning such party contained therein, but such party
is not responsible for the completeness or accuracy of information
concerning the other party unless such party knows or has reason to
believe that such information is inaccurate.
It is understood and agreed that a copy of this Agreement
shall be attached as an exhibit to the statement on Schedule 13G,
and any amendments hereto, filed on behalf of each of the parties
hereto.
Dated: February 14, 2006 Dated: February 14, 2006
T. ROWE PRICE NEW HORIZONS T. ROWE PRICE ASSOCIATES, INC.
FUND, INC.
By: /s/ Joseph A. Carrier By: /s/ Henry H. Hopkins
Joseph A. Carrier, Henry H. Hopkins,
Treasurer Vice President