UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 11-K

[ X ]

ANNUAL REPORT PURSUANT TO SECTION 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

For the fiscal year ended: December 31, 2006

Or

[    ]

TRANSITION REPORT PURSUANT TO SECTION 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

For the transition period from _______________ to ________________

Commission File Number 0-7275

      A. Full title of the plan and the address of the plan, if different from that of the issuer named below:

THE 401(k) STOCK PURCHASE PLAN
FOR EMPLOYEES OF CULLEN/FROST
BANKERS, INC. AND ITS AFFILIATES

      B. Name of issuer of the securities held pursuant to the plan and the address of its principal executive office:

CULLEN/FROST BANKERS, INC.

100 W. Houston Street

San Antonio, TX 78205

Telephone Number: (210) 220-4011

The 401(k) Stock Purchase Plan for Employees of

Cullen/Frost Bankers, Inc. and Its Affiliates

Financial Statements

and Supplemental Schedule

Years Ended December 31, 2006 and 2005

Contents

Financial Statements

   Report of Independent Registered Public Accounting Firm

3

   Statements of Net Assets Available for Benefits

4

   Statements of Changes in Net Assets Available for Benefits

5

   Notes to Financial Statements

6

Supplemental Schedule

   Schedule H, Line 4i - Schedule of Assets (Held at End of Year)

9

Report of Independent Registered Public Accounting Firm

Compensation and Benefits Committee of

  The 401(k) Stock Purchase Plan for Employees of

  Cullen/Frost Bankers, Inc. and Its Affiliates

We have audited the accompanying statements of net assets available for benefits of The 401(k) Stock Purchase Plan for Employees of Cullen/Frost Bankers, Inc. and Its Affiliates (the "Plan") as of December 31, 2006 and 2005, and the related statements of changes in net assets available for benefits for the years then ended. These financial statements are the responsibility of the Plan's management. Our responsibility is to express an opinion on these financial statements based on our audits.

We conducted our audits in accordance with the standards of the Public Company Accounting Oversight Board (United States). Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement. We were not engaged to perform an audit of the Plan's internal control over financial reporting. Our audits included consideration of internal control over financial reporting as a basis for designing audit procedures that are appropriate in the circumstances, but not for the purpose of expressing an opinion on the effectiveness of the Plan's internal control over financial reporting. Accordingly, we express no such opinion. An audit also includes examining, on a test basis, evidence supporting the amounts and disclosures in the financial statements, assessing the accounting principles used and significant estimates made by management, and 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 net assets available for benefits of the Plan at December 31, 2006 and 2005, and the changes in its net assets available for benefits for the years then ended, in conformity with U.S. generally accepted accounting principles.

Our audits were performed for the purpose of forming an opinion on the financial statements taken as a whole. The accompanying supplemental schedule of assets (held at end of year) as of December 31, 2006 is presented for purposes of additional analysis and is not a required part of the financial statements but is supplementary information required by the Department of Labor's Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974. This supplemental schedule is the responsibility of the Plan's management. The supplemental schedule has been subjected to the auditing procedures applied in our audits of the financial statements and, in our opinion, is fairly stated in all material respects in relation to the financial statements taken as a whole.

/s/ Ernst & Young, LLP

San Antonio, Texas

May 30, 2007

The 401(k) Stock Purchase Plan for Employees of

Cullen/Frost Bankers, Inc. and Its Affiliates

Statements of Net Assets Available for Benefits

December 31,

2006

2005

Assets

Participant-directed investments, at fair value

$

301,401,045

$

269,546,299

Receivables:

  Employer contributions

1,260,671

243,669

  Participants' contributions

387,191

347,428

Net assets available for benefits

$

303,048,907

$

270,137,396

See accompanying Notes to Financial Statements.

The 401(k) Stock Purchase Plan for Employees of

Cullen/Frost Bankers, Inc. and Its Affiliates

Statements of Changes in Net Assets Available for Benefits

Year Ended December 31,

2006

2005

Additions:

  Investment income:

    Interest

$

679,981

$

539,172

    Dividends

6,061,958

4,663,797

    Net appreciation in fair value of investments

17,120,127

20,214,121

23,862,066

25,417,090

  Contributions:

    Employer

8,718,677

6,857,785

    Participants

11,299,338

9,867,694

    Participant roll-overs

6,526,927

762,426

26,544,942

17,487,905

    Total additions

50,407,008

42,904,995

Deductions:

  Benefits paid to participants

17,305,422

10,413,152

  Administrative fees

190,075

164,583

    Total deductions

17,495,497

10,577,735

Net change

32,911,511

32,327,260

Net assets available for benefits:

  Beginning of year

270,137,396

237,810,136

  End of year

$

303,048,907

$

270,137,396

See accompanying Notes to Financial Statements.

The 401(k) Stock Purchase Plan for Employees of

Cullen/Frost Bankers, Inc. and Its Affiliates

Notes to Financial Statements

December 31, 2006 and 2005

1. Significant Accounting Policies

   Basis of Presentation. The accounting records of The 401(k) Stock Purchase Plan for Employees of Cullen/Frost Bankers, Inc. and Its Affiliates (the Plan) are maintained on the accrual basis of accounting. Benefits are recorded when paid.

   Investments. The Plan's investments are composed of common stock of Cullen/Frost Bankers, Inc. (CFBI), mutual funds, common collective trusts and loans to participants. Investments in CFBI common stock and mutual funds are stated at fair value based on quoted market prices on the valuation date. Investments in common collective trusts are stated at fair value as determined by the issuer based on the fair value of the underlying investments. Changes in fair market value and gains and losses on the sale of investment securities are reflected in the statements of changes in net assets available for benefits as net appreciation (depreciation) in fair value of investments. Loans to participants are valued at cost.

   Purchases and sales of securities are recorded on a trade-date basis. Dividends are recorded on the ex-dividend date. Interest is recorded on the accrual basis.

   Administrative Expenses. Certain administrative expenses of the Plan are paid by CFBI.

   Use of Estimates. The preparation of financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates that affect amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates.

   Reclassifications. Certain items in prior financial statements have been reclassified to conform to the current presentation. In that regard, participant roll-over contributions, which were previously reported as a component of participant contributions in 2005, have been reclassified and reported separately.

2. Description of the Plan

   The following is a general description of the Plan. Participants should refer to the Plan document for a more complete description of the Plan's provisions.

   General. The Plan is a defined contribution plan qualified under Section 401(a) of the Internal Revenue Code (IRC) and covers full-time employees who complete 90 consecutive days of service and part-time employees who complete 90 consecutive days of service and are scheduled to work more than 1,000 hours in a year. The Plan is subject to the provisions of the Employee Retirement Income Security Act of 1974 (ERISA).

   Contributions and Investment Options. Participants may contribute an amount not less than 2% and not exceeding 20% of their compensation, limited by 401(k) regulations, and may direct investments of their accounts into various investment options offered by the Plan. Participants are able to invest their contributions in these funds in 1% increments. Participants must contribute to the Plan to receive a CFBI matching contribution. CFBI matches 100% of each participant's contributions up to 6% of each participant's annual compensation. The match is initially invested in the common stock of CFBI. Each participant may elect to direct the investment of the matching contributions into other allowed investment options by electing to make investment transfers after such contributions are allocated to the participant's account.

   Participant Accounts. Each participant's account is credited with the participant's contributions and allocations of (a) CFBI's contributions and (b) plan earnings. The benefit to which a participant is entitled is the benefit that can be provided from the participant's account.

   Vesting. Participants are immediately vested in all contributions (both those made by the participant and by CFBI) plus actual earnings thereon.

   Participant Loans and Withdrawals. Participants may borrow from their fund accounts a minimum of $500 up to a maximum of $50,000, reduced by the highest amount of any loan outstanding within the previous twelve months, or 50% of their vested account balance. Loan terms range from 1 to 5 years or up to 30 years for the purchase of a primary residence. The loans are secured by the balance in the participant's account and bear interest at a rate commensurate with local prevailing rates. Principal and interest are paid ratably through semimonthly payroll deductions. Subject to Internal Revenue Service (IRS) limitations, participants may make hardship withdrawals from a portion of their 401(k) contributions to pay for an immediate and heavy financial need.

   Payment of Benefits. In the event of disability, retirement or death, a participant or the participant's beneficiary will receive a lump-sum payment equal to the amount of the participant's vested account in the Plan and all amounts that have been allocated to the participant's plan account. In the event of termination of employment for any other reason, the participant is entitled to the vested portion of the participant's account in the Plan and all vested amounts that have been allocated to the participant's Plan account. Terminated participants are required to take a distribution upon reaching normal retirement age (currently 65). Active participants have the option at age 70 1/2 as to whether or not they wish to begin minimum required distributions.

   Plan Termination. Although it has not expressed any present intent to do so, CFBI has the right under the Plan to discontinue its contributions at any time and to terminate the Plan subject to the provisions of ERISA.

3. Form 5500

   The following is a reconciliation of the Plan's net assets available for benefits per the financial statements to the amount reported in the Form 5500:

December 31,

2006

2005

Net assets available for benefits per the financial
  statements

$

303,048,907

$

270,137,396

Amounts allocated to withdrawing participants

(895,541

)

-

Net assets available for benefits per the Form 5500

$

302,153,366

$

270,137,396

   The following is a reconciliation of benefits paid to participants per the financial statements to the amount reported in the Form 5500:

December 31,

2006

2005

Benefits paid to participants per the financial statements

$

17,305,422

$

10,413,152

Amounts allocated to withdrawing participants at the end of the year

895,541

-

Amounts allocated to withdrawing participants at the end of the prior year

-

(685

)

Benefits paid to participants per the Form 5500

$

18,200,963

$

10,412,467

Amounts allocated to withdrawing participants are recorded on the Form 5500 for benefit claims that have been processed and approved for payment prior to December 31 but not yet paid as of that date.

4. Income Tax Status

   The Plan has received a determination letter from the IRS dated December 4, 2002, stating that the Plan is qualified under Section 401(a) of the IRC and, therefore, the related trust is exempt from taxation. The Plan has been amended subsequent to this determination by the IRS, however, the Plan administrator does not believe the amendments affect the Plan's tax-qualified status. Once qualified, the Plan is required to operate in conformity with the IRC to maintain its qualification. The Plan's sponsor has indicated that it will take the necessary steps, if any, to bring the Plan's operations into compliance in order to retain its tax-qualified status.

5. Investments

   The following presents individual investments that represent 5% or more of the Plan's net assets at year end:

December 31,

2006

2005

Cullen/Frost Bankers, Inc. common stock

$

175,273,845

$

167,994,604

   The Plan's investments (including gains and losses on investments bought and sold, as well as held during the year) appreciated in value during 2006 and 2005, as follows:

December 31,

2006

2005

Common collective trusts

$

8,409,239

$

3,613,447

Mutual funds

2,096,122

411,739

Common stock

6,614,766

16,188,935

$

17,120,127

$

20,214,121

6. Party-In-Interest Transactions

   Parties-in-interest are defined under Department of Labor regulations as any fiduciary of the Plan, any party rendering services to the Plan, the employer, and certain others. Accordingly, transactions conducted by the Trustee, Frost National Bank; the Plan administrator/custodian, AMVESCAP National Trust Company's retirement services division; and CFBI and its Affiliates, qualify as party-in-interest transactions.

   Plan assets are held and managed by the Trustee and the Plan administrator. The Trustee invests cash received, interest and dividend income as directed by the participants of the Plan. The Plan administrator also makes distributions to participants.

   Certain administrative functions are performed by employees of CFBI or its Affiliates; however, no such employees receive compensation from the Plan. Certain other administrative expenses are paid directly by CFBI.

7. Risks and Uncertainties

   The Plan provides for various investments in common stock, mutual funds, common collective trusts, and short-term investments. Investment securities, in general, are exposed to various risks, such as interest rate, credit, and overall market volatility risk. Due to the level of risk associated with certain investment securities, it is reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect the amounts reported in the statements of net assets available for benefits and participant account balances.

Supplemental Schedule

Cullen/Frost Bankers, Inc. and Its Affiliates

EIN: 74-1751768     Plan No.: 003

Schedule H, Line 4i - Schedule of Assets (Held at End of Year)

December 31, 2006

Identity of Issue,
Borrower, Lessor,
or Similar Party

Description of Investment, Including Maturity Date,
Rate of Interest, Collateral,
Par, or Maturity Date

Current
Value

Common Stock

*Cullen/Frost Bankers, Inc.

3,136,828 shares

$

175,273,845

*Participant Loans

Interest rates ranging from
4.00% - 8.25%; varying
maturity dates

11,889,012

Common/Collective Trusts

 Invesco Conservative Asset Allocation Trust 1

23,780 shares

669,171

 Invesco Moderate Asset Allocation Trust

230,085 shares

6,939,350

 Invesco Aggressive Asset Allocation Trust

116,782 shares

3,561,856

 Invesco 500 Index Trust

232,872 shares

8,779,286

*Frost EB Low Duration Fund

 95,581 shares

2,109,669

*Frost EB Kempner Deep Value Fund

230,035 shares

5,413,065

*Frost EB Fixed Income Fund

406,633 shares

5,926,031

*Frost EB Large Cap Core Stock Fund

246,920 shares

13,301,118

*Frost EB Small Cap Value Fund

256,657 shares

6,119,685

*Frost EB International Stock Fund

862,032 shares

13,945,529

66,764,760

Mutual Funds

  AIM Liquid Assets Fund (Money Market)

14,380,429 shares

14,380,429

  American Balanced Fund

442,525 shares

8,416,816

  American Beacon Large Cap Value Fund

357,013 shares

8,132,768

  Goldman Sachs Mid Cap Value Fund

194,469 shares

7,512,324

  American AMCAP Fund

343,875 shares

6,884,377

  Bridgeway Aggressive Investors II

  69,765 shares

1,181,115

  AIM Real Estate

  24,342 shares

818,626

  PIMCO Real Return

  13,800 shares

146,973

47,473,428

$

301,401,045

* Denotes party-in-interest.

Signatures

The Plan. Pursuant to the requirements of the Securities Exchange Act of 1934, the trustees (or other persons who administer the employee benefit plan) have duly caused this annual report to be signed on its behalf by the undersigned, hereunto duly authorized.

The 401(k) Stock Purchase Plan for Employees of

Cullen/Frost Bankers, Inc. and Its Affiliates

Date:

June 11, 2007

By:

  /s/ Emily Skillman

  Plan Administrator, Plan Chief Executive

  Officer and Plan Chief Financial Officer

(Duly Authorized Officer)

EXHIBIT INDEX

Exhibit

Number

Description

   23.1

Consent of Independent Registered Public Accounting Firm

   32.1

Section 1350 Certification