By ALFONSO CHARDY AND PABLO BACHELET

Frank Calzon, one of Washington's highest-profile exiled opponents of Fidel Castro who once was knocked out cold by a two-fisted blow to the head by a Cuban diplomat, considered his 20-something former chief of staff a model employee.

Felipe Sixto took care of the day-to-day workings of his Center for a Free Cuba as Calzon pursued his anti-Castro activism, urging foreign governments to take a tough stance on Havana and sending materials to dissidents on the island.

But nothing prepared Calzon for the realization that Sixto may have illegally pocketed hundreds of thousands of dollars in U.S. taxpayer money intended for efforts to propel democratic changes in Cuba.

Calzon, who says he felt a ''personal betrayal,'' expressed his outrage in a series of interviews with Miami Spanish-language radio stations and with The Miami Herald editorial board. He also embarked on what would become a successful effort to recover the money.

The amount of money involved in the alleged fraud has not been made public although government officials say it involves several hundred thousand dollars. Now, Calzon and others in the exile community are wondering how the mild-mannered Sixto, known as a can-do individual dealing with the tragedy of his wife's battle with cancer, went astray.

Three Republican lawmakers from Miami, Reps. Mario and Lincoln Diaz-Balart and Ileana Ros-Lehtinen, said they were ''deeply disturbed'' by the matter and urged a thorough investigation.

Sixto worked for over three years for Calzon before leaving for a White House job in June. Calzon called him ``perfect.''

With a degree in law from American University and a job as the White House liaison to the Cuban-American community, Sixto appeared to be on a fast track to important things. On March 1 he had been promoted to the position of special assistant to the president on intergovernmental affairs, which carries an annual salary of $97,000.

In late March, Sixto suddenly resigned as the Justice Department and U.S. Agency for International Development investigated allegations he misused an unspecified amount of U.S. grant money intended to promote democracy in Cuba.

Calzon, whom Havana has accused of being a CIA agent, realized something was wrong back in January, when he wanted to buy shortwave radios. His group has sent around 30,000 shortwave radios to Cuba over the last 10 years, so dissidents and others can get unfettered news about Cuba and the world.

Though Calzon was not specific about what drew his attention to the problem, he apparently found discrepancies in what the Center paid for the radios when Sixto handled the transactions and what they cost now.

Calzon said the financial discrepancies were not a one-time problem, but part of a pattern that stretched over three years -- almost the length of Sixto's employment.

Sixto resigned in late March, after he was contacted by the Center's lawyers.

''I want the U.S. government to get to the very bottom of this,'' said Calzon. ``I am confident that nobody else in the Center is involved.''

Calzon said the affair was doubly painful because he once thought Sixto could succeed him at the head of the Center for a Free Cuba. ``I'm 64 years old and at one point I'm going to retire. Who better than Felipe Sixto to run this thing, with his background?''

Sixto and his lawyer did not respond to repeated Miami Herald requests for comment.

Calzon is looking to pick up the pieces. His lawyer, Filiberto Agusti, says the Center is presenting USAID with a report of what happened.

Agusti says Sixto has returned most of the money, with the remainder to be paid over ``the next year or so.''

Without going into details, Agusti said the deal included ''some'' guarantees that Sixto would not face additional legal action from the Center.

The money was deposited in a trust account controlled by the Center's law firm, Steptoe and Johnson, until USAID decides if the Center can go ahead and use the money for its original purpose.