May 9 U.S. municipal bond funds attracted the most money in nearly 16 months, with $943.2 million of inflows spread evenly and broadly among sub-sectors in the $3.7 trillion market.
Investors have returned to muni bond funds this year, after fleeing last year in record numbers on concerns about financial crises in Puerto Rico and Detroit.
But the large inflows for the week ending May 7 haven't been seen since January 16, 2013, when the funds had $1.4 billion of inflows, according to data from Lipper, a Thomson Reuters unit.
Overall, total assets in muni bond funds are up about $19 billion, to $288.2 billion, since the beginning of the year.
"It was a huge turnaround," said Chris Mauro, director of municipal bond strategy at RBC Capital Markets.
"We usually look for certain sectors that dominate, or individual funds. That just wasn't the case here. There are no outliers," he said. "It's just a real, good, solid, clean report."
The strong results can be at least partly attributed to May 1 coupon payments. But the month of May last year had a similar level of coupon re-investment, without the same huge jump in inflows, he said.
(Reporting by Hilary Russ; Editing by Chizu Nomiyama)