Last week, investors did something they have not done in more than a year: They withdrew more money from municipal bond mutual funds than they put in.

Municipal funds reported a $187.2 million net outflow during the week ended March 31, according to Lipper FMI, the first net outflow since January 2009.

Based on the four-week moving average emphasized by Lipper, municipal funds are commanding cash from investors at the slowest pace in 15 months.

Anyone looking for a silver lining embedded in the latest number would have to dig pretty deep. The downturn was broad, with short-, intermediate-, and long-term flows all coming in far below their recent trends.

Further, the reversal appears confined to municipal funds. According to EPFR Global, bond funds overall had a stellar week, with emerging market, high-yield, and other U.S. taxable bond funds all reporting strong inflows.

Municipal funds, meanwhile, have clearly lost steam since their record-setting $77 billion cash intake last year.

The 590-fund, $470 billion municipal fund industry accumulated $14 billion in the first quarter of 2010. It commanded nearly $20 billion in the fourth quarter of 2009, and almost $27 billion in the third quarter.

Ron Schwartz, who manages more than $1 billion in municipal funds for RidgeWorth Capital Management, said some pullback was inevitable.

Since early 2009, a number of tailwinds have conspired to render short-term ­municipal bonds very expensive, Schwartz said.

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