Investors continue to stuff cash into municipal bond mutual funds, exacerbating a supply-demand imbalance that has kept yields on tax-exempt bonds near historic lows.

Municipal funds that report their figures weekly posted a net inflow of $760.6 million during the week ended Sept. 1, according to Lipper FMI. Weekly inflows have been positive nine straight weeks now. Investors have withdrawn money from municipal funds in only three weeks since the beginning of 2009.

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