(Bloomberg News) The biggest exchange-traded fund tracking the U.S. municipal debt market dropped the most in more than three years, pulled down after rallying to a record and the biggest premium ever to its underlying assets.

The iShares S&P National AMT-Free Bond Fund fell 2 percent to $111.45 on Thursday, the largest loss since October 2008. It was the fifth decline in the seven days since the fund, known as the MUB, traded at a record 3.96 percent premium to the per- share value of the bonds it holds. [Update: The fund has fallen further since then, and traded at a little more than $110 as of 10:15 Monday morning.]

The security fell following reports this week by Matt Fabian of Municipal Market Advisors and Timothy Strauts of Morningstar Inc., who noted that MUB's price had surged in relation to its holdings. Strauts recommended selling it.

"It's more a technical move than a reassessment," Fabian, a managing director at Concord, Massachusetts-based Municipal Market Advisors, said by e-mail yesterday. The fund "appears extremely overvalued," Fabian wrote in a Feb. 21 report.

Yesterday's slump drove the premium to net asset value down to 1.67 percent, the lowest since Jan. 20, according to data compiled by Bloomberg. The fund rose 0.1 percent to $111.56 at 9:36 a.m. New York time today. It has retreated 2.1 percent since climbing to a record high of $113.94 on Feb. 13.

The fund, with a market capitalization of $2.85 billion, had rallied 5.1 percent in 2012 through Feb. 22 amid declining bond sales by states and local governments that made the security one of the only ways to capture gains from the debt.

A net $218.5 million was invested in MUB this year, data compiled by XTF Inc. show. About $164.4 million was added in the past month, the eighth-highest inflow for U.S. fixed-income ETFs, data from the New York-based research firm show.

"It was at an unnatural premium," Strauts, a Chicago- based analyst at Morningstar, said by phone yesterday. "All the other muni ETFs were pretty much flat. Nothing special happened in the market. It seems solely focused on MUB."

Strauts said in his report that investors should buy the SPDR Nuveen Barclays Capital Municipal Bond ETF, known as TFI, which is trading closer to its net-asset value. TFI was unchanged at $24.17 yesterday and was at a discount of 0.06 percent on Feb. 22. The fund has seen inflows of about $72.3 million this year, of which $43.6 million was in the last month.

States and cities have reduced budgets as tax revenue stays below the peaks reached before December 2007, the start of the longest recession since the 1930s. Issuance dropped to $258.3 billion in 2011, the least in eight years, according to data compiled by Bloomberg.

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