The largest U.S. exchange-traded fund tracking municipal bonds touched the highest price since it began five years ago as investors speculated that tax rates will increase to help curb the federal deficit.

The iShares S&P National AMT-Free Municipal Bond Fund traded at $114.07 at about 1:30 p.m. today in New York, reaching as high as $114.52. It’s the steepest price since it started in September 2007. The ETF, known as MUB, has $3.5 billion of assets. It has been trading at a premium to its underlying assets since July 16, the longest stretch since March, data compiled by Bloomberg show.

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