Announcing its second acquisition deal in less than a week, UST Corp. of Boston said Monday that it had agreed to buy Affiliated Community Bancorp of Waltham, Mass., for $259 million in stock.

Affiliated, a $1.1 billion-asset thrift, would bolster UST's market position in Middlesex County, a string of affluent suburban communities west of Boston that UST executives deem critical for expansion.

In another move on Middlesex County, UST said last week that it would purchase $520 million-asset Somerset Savings Bank of Somerville for $93.7 million.

Pursuing what chairman and chief executive officer Neal F. Finnegan called an "assembly strategy," UST has announced or completed five acquisitions that would triple its asset size in 18 months. The company has $3.7 billion of assets, $2.8 billion of deposits, and 66 branches.

It is focusing that growth and acquisition strategy on eastern Massachusetts and especially Boston, having sold a four-branch, $110 million-asset Connecticut subsidiary last December to New Jersey-based Hubco Inc.

Mr. Finnegan said UST wants to be Boston's "other big bank," behind BankBoston Corp. and Fleet Financial Group.

UST aims to compete against the local leaders in the small-business, middle-market, and consumer categories.

UST is "trying to become a viable alternative to the two dominant players in the area, and I think they are getting there," said Kevin T. Timmons, an analyst at First Albany Corp.

When the two most recently announced acquisitions are consummated, UST would be the fourth-largest commercial bank in the eastern part of the Commonwealth, behind BankBoston, Fleet, and State Street Corp., analysts said.

The two purchases would boost UST's assets by 43%, to $5.4 billion, and its deposit base by 40%, to $4 billion. UST would have 83 branches, 44 in Middlesex County.

Mr. Finnegan said more acquisitions are ahead.

"It is incumbent on us, if we can, to build out from Boston and then to the suburbs," he said in a telephone interview. "Then we can think about widening our circle."

The Affiliated deal, which is scheduled to close in the first quarter, provides for a tax-free exchange of 1.41 shares of UST's stock for each share of Affiliated, or $39.42 in UST stock based on Friday's closing price.

At 2.3 times Affiliated's book value and 20 times earnings, the premium is in line with recent acquisitions in the region, analysts said. The Somerset Savings acquisition, at $5.63 a share, was valued at 2.5 times book.

UST said it would take a $12 million pretax charge in the first quarter for merger-related expenses, on top of a previously anticipated $7.5 million charge related to Somerset. Layoffs are expected to be minimal, because the acquired branches would not overlap with UST locations, Mr. Finnegan said.

Affiliated is the holding company for Lexington Savings Bank, Federal Savings Bank, and Middlesex Bank and Trust Co., which have a combined 12 branches in Middlesex County.

Affiliated's president and chief executive officer, Timothy J. Hansberry, would become president and chief operating officer of UST's principal subsidiary, USTrust, and vice chairman and chief operating officer of the parent company.

Mr. Hansberry said the deal would give his customers "access to a number of additional banking services, including investment management and trust" and, for business customers, "cash management, asset based lending, international services, and leasing."

Somerset Savings Bank has four branches in Somerville and one in Burlington, Mass.

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