It is not clear how UBS will decide which clients to unmask, though American officials have said that they are interested only in the biggest accounts — some containing hundreds of millions of dollars — and accounts that made use of offshore entities and sham corporations.

The agreement is another victory for the I.R.S. and the Justice Department in the long-running case against UBS, which in February paid $780 million and admitted to criminal wrongdoing in selling offshore banking services that enabled tax evasion. Larger Swiss banks like UBS and Credit Suisse have curtailed banking services for wealthy Americans in response to the increased scrutiny.

Some smaller, centuries-old private Swiss banks, however, are stepping up their efforts to attract American money, given the importance of foreign clients to the nation’s financial institutions.

Executives at smaller Swiss banks and trade groups say they are increasingly working with Swiss financial and legal companies to set up offshore entities as a way to shield assets from prying regulatory eyes. They have also been reassuring clients that their accounts will remain confidential.

Sebastian Dovey, a managing partner at Scorpio Partnership, a wealth consulting firm in London , said that “we have not seen huge amounts of movement” of American money out of Swiss banks.

The current penalty for this type of failure to disclose assets is up to 50 percent of the highest annual balance of each account for each of the last three years — an amount that can quickly wipe out an investor and still leave him owing taxes and interest.

Investors who come forward before Sept. 23 face a reduced penalty, of 5 percent to 20 percent, depending in part on whether the wealth was inherited. They will also be hit with the penalty just once, on the highest balance in the accounts during the last six years.

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The Justice Department has opened criminal investigations of 150 UBS clients, and is likely to bring more indictments on top of the four it brought in recent months. Only clients who are prosecuted are likely to have their names become public.

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The landmark settlement is expected to provide a road map for the I.R.S. as it tries to clamp down on tax evasion by Americans who use offshore accounts.

Smaller Swiss banks are still helping clients hide billions of dollars through complex structures in offshore havens in the Caribbean , Panama , Luxembourg , Singapore and Guernsey in the English Channel. The crackdown is casting a spotlight on the network of lawyers, accountants and advisers who help underpin the Swiss private banking industry by steering clients to such banks.

An American official, who spoke on condition of anonymity because he was not authorized to speak publicly, said this week that the Justice Department was preparing criminal cases against some of the Swiss agents and intermediaries who set up offshore entities for clients and funnel their money to private banks. They take a referral fee in the process — sometimes up to 50 percent, according to a former UBS private banker.

A little-known I.R.S. program, the Offshore Identification Unit, is helping to build a map of this world. It recently began tracking all disclosures of wealthy Americans who, unnerved by the UBS situation, have come forward to declare their assets and account details, often naming the private banks and intermediaries they used.

Glimpses of the network were revealed last month when one UBS client, Jeffrey P. Chernick, of Stanfordville , N.Y., pleaded guilty to criminal charges of evading taxes on $8 million he put into UBS accounts in the Cayman Islands and Zurich . Mr. Chernick worked with and received advice from a Swiss lawyer and a bank executive at a small private bank in Zurich, neither of them UBS employees, according to court records.

Many intermediaries still extol the advantages of Swiss accounts, including Swiss political and economic stability, its European base and its high level of client service — and the fact that Switzerland has not traditionally considered tax evasion to be a crime.

Micheloud & Cie, which has offices in Lausanne and other Swiss cities, offers to help clients set up accounts at the biggest private banks in Switzerland. Its Web site describes such accounts as “the ultimate symbol of wealth, personal achievement and privacy,” and advises clients on how to set up numbered accounts with code names like “Octopussy” and “Cello.”

The firm says it will not accept clients who are citizens of various countries that it considers undesirable, including Albania , Uzbekistan , Nigeria — and now the United States . But it also assures prospective clients that “Swiss bank secrecy is not lifted for tax evasion, even upon the request of a foreign government.”

Other countries have also pressured Switzerland in recent months for names of suspected tax cheats, including Germany and Italy . The attention has left Swiss bankers perturbed about demands that they ensure that their clients pay their taxes.

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