WASHINGTON -- U.S. citizens who defy restrictions on travel to Cuba increasingly are returning stateside to find an unexpected souvenir: A letter from the feds demanding they pay $7,500 or so in fines.

The number of such penalty letters has suddenly spiked, and unsuspecting U.S. travelers are yelping in surprise at the potential cost of their travels.

``I think it's very stupid,'' said Donna, a 64-year-old retired social worker in Chicago who asked that her last name not be used. After a bike trip to Cuba, she got notice in June that the Treasury Department plans to levy $7,650 in fines against her. ``They should leave people like me alone who do no harm.''

From May 4 to July 30, a division of the U.S. Treasury Department that monitors travel to Cuba sent out 443 letters seeking average fines of $7,500 -- a sharp increase from the 74 letters mailed from Jan. 3 to May 3.

Those receiving penalty letters include New York City high school students and teachers, scuba divers, cyclists, a Massachusetts bird watcher, a Santeria buff from the Pacific Northwest -- a panoply of Americans intrigued by the tropical communist bastion of President Fidel Castro and willing to wriggle under the legal trip wire. Interest in Cuba has surged despite -- or perhaps because of -- a longstanding law that forbids U.S. citizens from spending money on the island.

The travel restrictions are now in roiling waters as the White House and Congress veer in sharply different directions on policy toward Cuba. Staking out a hard line, President Bush pledged July 13 to detect and punish those who visit Cuba illegally ``to the fullest extent with a view toward preventing unlicensed and excessive travel . . . ''

A majority of the U.S. House, meanwhile, wants to facilitate travel to Cuba. On a 240-186 vote, the House on July 26 denied the executive branch any funds to enforce the travel restrictions. The measure now heads to the Senate, where observers say it could pass in the autumn.

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