Moody's Investors Service Inc. upgraded the senior debt ratings of Bank of Boston Corp. and Shawmut National Corp., another sign that the banks are on the mend.

The upgrade boosts Bank of Boston's senior debt rating to Baa3 from Ba2, the first time the company's debt has been rated investment grade in two years.

Shawmut's senior debt ratings were raised to Ba1 from Ba3.

Separately, Moody's affirmed the A2 senior debt rating of Comerica Inc. and raised the subordinated debt rating of its merger partner, Manufacturers National Corp., to A3 from Baa1. The company are scheduled to complete their merger today.

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