William J. Shea, a vice chairman of BankBoston Corp. who helped engineer its merger with BayBanks Inc., resigned abruptly on Thursday.

Mr. Shea, 49, a five-year veteran who was also chief financial officer and treasurer, said he would remain at the bank for an unspecified period to ensure a smooth transition. But his CFO duties were immediately handed off to Susannah M. Swihart, 41, an executive vice president.

In a telephone interview, Mr. Shea said he is stepping down because he wants "to run a medium-size company, maybe something entrepreneurial." He said he has not ruled out other jobs in financial services.

Observers of the bank said they were surprised by the departure. "He's been a fixture there," said John Carusone, president of the Bank Analysis Center in Hartford, Conn. "He has been key to their recovery in the early 1990s, to their expansion, and to their strategic initiatives."

Before joining BankBoston in 1992, Mr. Shea was a senior partner and member of the firm council at Coopers & Lybrand in Boston.

Ms. Swihart has been with the bank and its predecessor for 17 years, the last two of them as chief of staff.

She will be responsible for implementing the bank's corporate strategy, dubbed managing for value.

Before that, she held various executive positions in the corporate bank, including group executive for specialized lending and for Bank of Boston's media and communications groups. Ms. Swihart was also a vice chairman of the bank's senior credit committee.

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