NEW YORK, MAY 24 -- Admitted inside trader Ivan Boesky testified today that he deducted half of the $100 million penalty he paid in a government plea bargain from his personal income taxes.

Boesky, who also turned government informant and served prison time as part of his plea agreement, wrote the $50 million off as a capital expenditure on his 1986 tax return, according to his attorney, Charles Davidow.

The revelation came at the securities fraud trial of John Mulheren, where Boesky appeared as the government's star witness. Mulheren, accused of helping Boesky evade taxes and securities laws in exchange for valuable investment tips, was implicated when Boesky agreed to cooperate with the government to settle inside trading charges.

U.S. District Judge Miriam Goldman Cedarbaum allowed the jury to hear about Boesky's tax deduction, saying, ''It only goes to show that the $100 million was not really as painful or expensive as it appears.''

Of the money he paid the government, $50 million represented ill-gotten gains and was taken as a tax deduction, Boesky testified. That money was placed in a fund to repay victims of his crimes. The other $50 million was a penalty and not deducted, his attorney said.

Mulheren, 50, the first of those implicated by Boesky to challenge the charges in trial, watched Boesky intently during his testimony today, smiling occasionally. He faces 41 counts of fraud and conspiracy.

Defense attorney Thomas Puccio hammered away at Boesky's financial dealings, trying to determine what became of Boesky's fortune before his 1987 sentencing, when his attorneys said he was nearly wiped out.

Boesky claimed his legal ordeal has nearly bankrupted him.

''You're the kind of man who was concerned about how much money you had or didn't have?'' Puccio asked. Boesky replied: ''Less than you might think. It has far less ... consequence than you might think.''

When Puccio referred to the fine as ''only'' $50 million, Boesky interrupted him to say, ''Don't minimize that amount, sir. ... It is not only $50 million,'' Boesky added, breaking into a rare smile. ''I think it was a record.''

Boesky, 53, pleaded guilty to a securities law violation and also agreed to pay the $100 million to settle civil charges of trading on nonpublic information. He was sentenced to three years in prison and was released last month from a Brooklyn halfway house.

On the witness stand, he carefully skirted a question as to whether he revealed all his crimes to government investigators in his plea bargain.

''I have told the government 100 percent of what they wanted to know,'' he said.