"They might wait a couple of days to see how things shake out before they decide what they are going to do, and they also need time to talk to their financial advisers," he said.

Investors pulled about $14.5 million out of municipal-bond mutual funds on Tuesday, the seventh straight day of flows from the funds, according to data from TrimTabs Investment Research. There was about $861 million in total withdrawals from U.S. municipal-bond mutual funds in the week ended Aug. 3, before the downgrade, and that was the greatest weekly outflow since April, according to Lipper US Fund Flows.

Fund Withdrawals

Mutual-fund investors took money out of the funds for six straight months beginning in November 2010, according to data from Chicago-based Morningstar Inc., including $13 billion in outflows in December, the month when analyst Meredith Whitney forecast that there could be 50 to 100 "sizable" municipal- bond defaults. From May through July investors were adding money to the funds, Morningstar said.

"There's no parallel," in current fund flows to the outflows municipal-bond funds experienced in late 2010, said Spalding.

Municipal bonds fell 0.04 percent this week through yesterday, as measured by the Bank of America Merrill Lynch Municipal Master Index. Prerefunded municipal bonds, or those backed by Treasury bonds held in escrow, gained 0.07 percent, as measured by the Bank of America Merrill Lynch Municipal Prerefunded Index.

Some investors may be holding on to municipal-bond investments because there are few other safe havens available, said Robert Kane, founder and chief executive officer for BondView, which provides bond-market data for advisers and individual investors.

"There's a kind of tug-of-war going on here with people wanting muni bonds and thinking longer term, and people thinking 'Gee, I want to get out,'" he said. "But if you do get out, where do you go?"

Signs Of Selling

There were some signs of selling following the U.S. downgrade. Shares of the iShares S&P National AMT-Free Bond Fund, an exchange-traded fund, closed Monday at a 1.77 percent discount to its net asset value, or the underlying value of its holdings, the largest discount for the fund since December 2010, according to data compiled by Bloomberg.

That discount may be because shares of the exchange-traded fund are more liquid than bonds sold over-the-counter, so market movements may show up in its shares before they are reflected in prices of the underlying bonds, said Matt Tucker, head of fixed- income investment strategy for BlackRock Inc.'s IShares unit. On Wednesday the shares closed at a 0.77 percent discount to net asset value.