Seven years ago, Robert Hanawalt ditched a sales career in Washington to move to Prague, where he quickly realized that he could live indefinitely without official paperwork.

He taught English illegally for four years on 90-day tourist visas. The trick? Quick trips over the border, which reset the clock with a fresh passport stamp.

"I did that," Mr. Hanawalt says. "But after the first few times I thought, 'Why even bother? Nobody is checking these things.' "

But as nine countries, including the Czech Republic, join the European Union's borderless Schengen zone Friday, Brussels is now ordering member states to get tough on visa policy.

That could spell trouble for an unlikely class of illegal immigrants: American expats. Attracted by English teaching jobs, the low cost of living, and societies just waking up to the possibilities of Western tourism, thousands are estimated to be living and working illegally in central and eastern Europe.

Prague quickly became an expatriate magnet. Today, 5,000 Americans are registered with the US Embassy here, though there's no official tally of the total number of Americans living in the Czech Republic. Local media estimate it to be nearly 20,000.

Brussels is taking aim at such visa riders. Now, Americans and Canadians can initially travel visa-free to Schengen countries for up to 90 days. But if at the end of that time they want to stay, they must go somewhere outside the zone – Ukraine or Montenegro, for example – to apply for a long-term visa.

Many expats are wondering what to do now, having set down roots here.

"There is definitely some panic about Schengen," says Evan Rail, a travel writer who has lived in Prague for eight years, but has been "riding a tourist visa" for the last two.

Hanawalt has gotten a valid residency permit and runs a business helping other Americans in Prague negotiate the country's immigration bureaucracy and get legal themselves. Mark Wright, who has been teaching English illegally in Prague for two years, found another teaching job at a language school that says it will help him obtain a visa.

Other Americans are applying for Czech business licenses, another avenue to obtaining a residency visa. But some are taking their chances.

"Unfortunately one can't go up to a government official and ask exactly how much harder it will be to live here illegally in the new year," says Mr. Wright. "It's possible enforcement might not change at all, and I know some expats who are banking on it."

The Czech interior ministry is promising increased enforcement. Spokesman Vladimir Repka wrote in an e-mail this week that in 2007 more than 4,000 people were deported for visa violations, though he did not know how many were Americans.

Schengen's expansion is affecting others as well. Ukrainians, long accustomed to unfettered travel to Poland, now need a visa even for day trips. Slovenia is closing down unmanned footbridges along its border with Croatia.

Not every American in Prague is greeting Schengen coldly. "As someone living here legally, I think it's only fair that some of the permanent tourists here be made to do the same thing," says Mark Anderson, who moved here six years ago and started his own cleaning business.