The Flushing offices of Mon Leang Mui & Company weren’t much to behold. Mr. Mui, who billed himself as a certified public accountant, had a lone desk and two idle-looking employees.

Still, prosecutors say, Mr. Mui had a knack for manipulating clients, including an immigrant couple that ran a Chinatown garment business. When the couple learned they had tax trouble in the early 1990s, they visited Mr. Mui.

That visit, the chief assistant U.S. attorney, Andrew Hruska, told a Brooklyn jury yesterday, launched a long financial nightmare. Mr. Mui allegedly stole $400,000 from the couple and used the Internal Revenue Service and his mother to mask his theft.

According to prosecutors, Mr. Mui convinced the couple, Yee Sing Tung and Mee Ying Tung, that they had to give him cashiers checks to cover large tax debts. They say that Mr. Mui, who was not a certified accountant, used the Tungs’ checks to vastly overpay his own mother’s tax bill, then reap the resulting tax refund to pay his own debts.

The Tungs, meanwhile, lost their money and their home, the prosecutors say.

Mr. Mui “committed a cunning fraud and then went to elaborate lengths to cover it up,” Mr. Hruska said during opening statements yesterday.

“He took advantage of some unsophisticated clients,” the prosecutor said, “and basically managed to launder money through the IRS.”

A lawyer for Mr. Mui, 47, reserved comment to jurors until the start of the defense case.

“We have a lot to say and I think you’ll see the story unfold,” the defense lawyer, M. Bradford Randolph, said after court. “The evidence will show the defendant is not guilty.”

Mr. Mui is charged with obstructing the administration of tax laws, evading income taxes, and making false statements.

Prosecutors say that Mr. Mui faces at least four-and-half-years in prison if he is convicted.

Testifying yesterday, Mr. Tung said that he and his wife first visited Mr. Mui in 1990 after getting a letter from the IRS inquiring about taxes on their business, May Main Sports.

The Tungs, who have lived in America for 30 years, had worked up from menial jobs to owning a skirt assembly business. Mr. Tung testified that he and his wife were paying the workers in cash – and not reporting their accurate payroll to the government.

Mr. Tung said he and his wife hesitated to hire Mr. Mui, partly because no one seemed to be working at his office. But when Mr. Mui warned that they could go to jail, they retained him.

Prosecutors say that Mr. Mui cheated the couple twice, once in 1990 and again in 1992. According to prosecutors, Mr. Mui got the Tungs to write checks payable to the IRS for the supposed back tax bills. In reality, prosecutors say, the checks went into the IRS account of Mr. Mui’s mother, Fong Oi Chan, who then got IRS refunds.

“He said ‘You’re very lucky, I took care of all your problems,’ ” Mr. Tung testified.

The couple was still left with a tax problem and eventually “got wise” to the scheme, Mr. Hruska said. The Tungs sued Mr. Mui in state court but lost.

Mr. Mui is accused of lying to IRS investigators in the late 1990s. He allegedly denied that Ms. Chan, who died in 1997,was his mother. Mr. Hruska also accused Mr. Mui of lying about being a certified accountant and of not paying his own taxes for 20 years.