The New York Stock Exchange halted trading on both Ambac Financial Group Inc. and Radian Group Inc. under its "sub-penny" rules yesterday after shares of both insurers fell below $1.05.

The exchange stops trading on a security when it is - or is "imminently likely" - to trade below $1.00. The sub-penny halt is automatically triggered when a shares of a stock fall below $1.05, according to NYSE rules.

Shares of Ambac finished the day at $1.16, down $0.02 from open. Shares of Radian fell to $0.77 , down $0.32 at close.

Trading on the securities can resume when the stock has traded at or above $1.10 per share for a full day on another automated trading center.

Along with other bond insurers caught in the credit crunch, shares of the two companies have plunged this past year. Less than a year ago, Ambac traded at a 52-week high of $88.41, while Radian traded at 52-week high of $55.96. Both were removed from the Russell 1000 large-cap index last Friday.

MBIA Inc. closed yesterday at $3.97 compared to a 52-week high of $68.98, Shares of Security Capital Assurance Ltd. closed at $0.29 yesterday off their 52-week high of $31.32. Bond insurer ACA Holding Inc. was removed from the list by the NYSE last December.

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