Fidelity Investments and other asset managers are telling U.S. clients who live outside the country that they can no longer buy or trade mutual funds in their brokerage accounts.

Stephen Austin, a spokesman for the financial-services firm, said the change, effective Aug. 1, was prompted by "today's continually evolving global regulatory environment," but he said it wasn't in response to a specific issue.

The change will affect about 50,000 accounts, or less than 0.3% of Fidelity's 20 million accounts, he said.

"Customers will not be forced to sell holdings simply because they live in a foreign country," Mr. Austin said.

Observers said fund managers are becoming more conservative in the wake of global developments such as the U.S. Foreign Account Tax Compliance Act and other U.S. efforts.