The number of companies participating in the Federal Deposit Insurance Corp.'s temporary debt guarantee coverage program increased by more than a third last month.

Ninety-seven issuers were participating at the end of March, versus 73 at the end of February. The amount of outstanding debt guaranteed by the agency rose 25%, to $336 billion.

The largest increase in participation was among institutions with assets of less than $10 billion. There were 18 more issuers in that category in the month, bringing the total participating to 46. Participation by larger institutions rose by just two, to 20 issuers.

The voluntary program was launched in October as part of a broader effort to safeguard the banking industry. Participants pay a premium to guarantee their senior unsecured debt, and the protection can last until the end of 2012. Under another component of the program, banks can choose temporary protection of their no-interest checking deposits.

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