UNITED STATES

OMB APPROVAL

SECURITIES AND EXCHANGE COMMISSION

OMB Number:
3235-0145

Washington, D.C. 20549

Expires:
December 31, 2005

SCHEDULE 13G

Estimated average
burden hours per
response. . 11

o

Rule 13d-1(b)

ý

Rule 13d-1(c)

o

Rule 13d-1(d)

CUSIP No.  90385V107

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Discovery Equity Partners, L.P.

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

Not Applicable.

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
Illinois

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
None.

6.

Shared Voting Power 
937,315 Shares

7.

Sole Dispositive Power 
None.

8.

Shared Dispositive Power
937,315 Shares

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
937,315 Shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o
Not Applicable

11.

Percent of Class Represented by Amount in Row (9)
5.7 %

12.

Type of Reporting Person (See Instructions)
PN

CUSIP No.  90385V107

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Discovery Group I, LLC

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

Not Applicable.

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
None.

6.

Shared Voting Power 
1,105,830 Shares

7.

Sole Dispositive Power 
None.

8.

Shared Dispositive Power
1,105,830 Shares

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,105,830 Shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o
Not Applicable

11.

Percent of Class Represented by Amount in Row (9)
6.7 %

12.

Type of Reporting Person (See Instructions)
OO

CUSIP No.  90385V107

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Daniel J. Donoghue

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

Not Applicable.

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
U.S.A.

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With 

5.

Sole Voting Power
None.

6.

Shared Voting Power 
1,105,830 Shares

7.

Sole Dispositive Power 
None.

8.

Shared Dispositive Power
1,105,830 Shares

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,105,830 Shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o
Not Applicable

11.

Percent of Class Represented by Amount in Row (9)
6.7 %

12.

Type of Reporting Person (See Instructions)
IN

CUSIP No.  90385V107

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Michael R. Murphy

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

Not Applicable.

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
U.S.A.

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With 

5.

Sole Voting Power
None.

6.

Shared Voting Power 
1,105,830 Shares

7.

Sole Dispositive Power 
None.

8.

Shared Dispositive Power
1,105,830 Shares

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,105,830 Shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o
Not Applicable.

11.

Percent of Class Represented by Amount in Row (9)
6.7 %

12.

Type of Reporting Person (See Instructions)
IN

Item 1.

(a)

Name of Issuer
Ultra Clean Holdings, Inc.

(b)

Address of Issuer’s Principal Executive Offices
150 Independence Drive, Menlo Park, California 94025-1136

Item 2.

(a)

Name of Person Filing

Discovery Equity Partners, L.P. (“Discovery Partners”)

Discovery Group I, LLC, the general partner of Discovery Partners (“Discovery Group”)

Daniel J. Donoghue, a Managing Member of Discovery Group

Michael R. Murphy, a Managing Member of Discovery Group

(b)

Address of Principal Business Office or, if none, Residence

Discovery Partners, Discovery Group, Mr. Donoghue, and Mr. Murphy are all located at:

Hyatt Center, 24^th Floor, 71 South Wacker Drive, Chicago, Illinois 60606

(c)

Citizenship

Discovery Partners is an Illinois limited partnership

Discovery Group is a Delaware limited liability company

Mr. Donoghue and Mr. Murphy are U.S. citizens

(d)

Title of Class of Securities
Common Stock, par value $.001 per share

(e)

CUSIP Number
90385V107

Item 3.

If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

Not Applicable.

(a)

o

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b)

o

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).

(c)

o

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).

(d)

o

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C 80a-8).

(e)

o

An investment adviser in accordance with §240.13d-1(b)(1)(ii)(E);

(f)

o

An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F);

(g)

o

A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h)

o

A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i)

o

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

o

Group, in accordance with §240.13d-1(b)(1)(ii)(J).

Item 4.

Ownership

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

(a)

Amount beneficially owned:

Discovery Partners – 937,315
Discovery Group – 1,105,830
Mr. Donoghue – 1,105,830
Mr. Murphy – 1,105,830

(b)

Percent of class:

Discovery Partners – 5.7%
Discovery Group – 6.7%
Mr. Donoghue – 6.7%
Mr. Murphy – 6.7%

The foregoing percentages are based on 16,437,650 shares of Common Stock of the Issuer identified in Item 1 outstanding as of October 31, 2005, as reported in the Issuer’s Quarterly Report on Form 10-Q for the quarterly period ended September 30, 2005.

(c)

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote

None

(ii)

Shared power to vote or to direct the vote

Discovery Partners – 937,315
Discovery Group – 1,105,830
Mr. Donoghue – 1,105,830
Mr. Murphy– 1,105,830

(iii)

Sole power to dispose or to direct the disposition of

None

(iv)

Shared power to dispose or to direct the disposition of

Discovery Partners – 937,315
Discovery Group – 1,105,830
Mr. Donoghue – 1,105,830
Mr. Murphy – 1,105,830

Item 5.

Ownership of Five Percent or Less of a Class

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following   o.

Not Applicable.

Item 6.

Ownership of More than Five Percent on Behalf of Another Person

The shares reported herein have been acquired on behalf of discretionary clients of Discovery Group, including Discovery Partners.  Such discretionary clients are entitled to receive all dividends from, and proceeds from the sale of, those shares.  Except for Discovery Partners, none of those discretionary clients, to the knowledge of Discovery Partners, Discovery Group, Mr. Donoghue, or Mr. Murphy, has an economic interest in more than 5% of the class.

Item 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person

Not Applicable.

Item 8.

Identification and Classification of Members of the Group

Not Applicable.

Item 9.

Notice of Dissolution of Group

Not Applicable.

Item 10.

Certification

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

February 7, 2006

Date

DISCOVERY GROUP I, LLC,

for itself and as general partner of

DISCOVERY EQUITY PARTNERS, L.P.

/s/ Michael R. Murphy

Signature

Michael R. Murphy, Managing Member

Name/Title

/s/ Daniel J. Donoghue

Signature

Daniel J. Donoghue

Name/Title

/s/ Michael R. Murphy

Signature

Michael R. Murphy

Name/Title

Exhibit 1

Joint Filing Agreement dated as of February 7, 2006, by and among Discovery Equity Partners, L.P., Discovery Group I, LLC; Daniel J. Donoghue; and Michael R. Murphy.