By Joan Gralla

NEW YORK (Reuters) - Berkshire Hathaway Inc's (BRKa.N: Quote, Profile, Research) new municipal bond insurance arm on Friday captured its second top rating, this time from Moody's Investors Service, which cited its fast growth despite a credit crunch.

By late April, the new bond insurer had guaranteed more than $8 billion of municipal bonds, based on the net par amount, Moody's analyst James Eck told Reuters by telephone.

Moody's, like Standard & Poor's Ratings Services, gave the new company, called the Berkshire Hathaway Assurance Corporation, its highest "AAA" rating.

It was only in late December that billionaire investor Warren Buffett created the bond insurer, partly at the request of New York's insurance superintendent Eric Dinallo, who was trying to stabilize the troubled insurers that guarantee about half of the $2.6 trillion of municipal bond market.

Several insurers faced downgrades because they expanded into risky mortgage-linked securities, and investors were fleeing the debt they guaranteed, seeking to avoid losses.

"Berkshire Hathaway Assurance Corporation has rapidly grown its insured portfolio of U.S. public finance credits during a period of great stress in the credit markets," Eck said in a statement.

"The company's sound financial profile and its affiliation with Berkshire give it a meaningful presence in the financial guaranty insurance marketplace," he added.

A spokeswoman for Berkshire was not available to say whether the company had guaranteed new issues of municipal bonds in addition to the outstanding debt it began backing or whether it was capturing the high premiums Buffett said he would demand.