David Finnerty, a former specialist at Fleet Specialist Inc., made thousands of improper trades at the New York Stock Exchange for his firm’s account, thus enriching himself through inflated bonuses, prosecutors argued Wednesday as his fraud case drew to a close.

In his closing statement, Assistant U.S. Attorney Anirudh Bansal said that Mr. Finnerty, who was the specialist for General Electric Co., one of the Big Board’s most active stocks, improperly positioned himself between public buy and sell orders more than 26,000 times — sometimes dozens of times a day — between 1999 and 2003.

His firm made $4.5 million in illegal profits as a result of the trades and Mr. Finnerty personally received hundreds of thousands of dollars in inflated bonuses based on those illegal profits, Mr. Bansal said.

Frederick Hafetz, Mr. Finnerty’s lawyer, countered Wednesday that the government failed to prove that Mr. Finnerty had intended to commit a crime and that he would have been “insane” to do so, given the electronic monitoring present on the NYSE floor.

“It is indeed troubling that a man entrusted with the New York Stock Exchange’s flagship stock would abuse that trust for his own personal profit,” Mr. Bansal said.

Mr. Finnerty, who is accused of making improper trades in GE and two biotech stocks, is charged with three counts of securities fraud and faces a maximum of 20 years in prison. The case is expected to go to the jury later Wednesday.

His case is an important one for the government, which has recently been forced to drop criminal cases against two former specialists and two others were acquitted of securities-fraud charges.

Prosecutors have brought criminal charges against 15 former specialists for allegedly making improper trades at the New York Stock Exchange. Two pleaded guilty in May and two were convicted of securities fraud in July.

Specialists match buyers and sellers at the NYSE, a unit of the publicly traded NYSE Group Inc., and provide liquidity by buying or selling shares when there is an imbalance on the floor.

Mr. Bansal, the prosecutor, argued Wednesday that Mr. Finnerty knew what he was doing was wrong and abruptly stopped improperly stepping between public buy and sell orders in late 2002 when the NYSE indicated it was probing those types of trades.

The prosecutor said that Mr. Finnerty, when confronted by a clerk about whether a trade was proper, simply pointed to his badge and told the clerk that he was the specialist and to execute it. He also tried to intimidate another clerk when that clerk was asked to testify as part of the NYSE’s probe into trading, Mr. Bansal said.

Mr. Bansal said the trades were intentional and far from mistakes on the part of Mr. Finnerty or his clerks.

“A mistake like that doesn’t happen 26,000 times, a mistake that just happens to put money in someone’s pocket,” Mr. Bansal said.

In his closing statement, Mr. Hafetz, Mr. Finnerty’s lawyer, said Mr. Finnerty didn’t believe he did anything wrong and was acting in “good faith” in his time at the Big Board.

Mr. Hafetz said Mr. Finnerty traded in the open at the NYSE and made no efforts to conceal his activities. Given the electronic monitoring of trading at the NYSE, Mr. Hafetz said Mr. Finnerty was essentially in an “electronic goldfish bowl” and it would make “no sense” for him to try to commit a fraud there.

“It is like the San Andreas fault running through the government’s case,” Mr. Hafetz said. “It’s like a barrier they cannot overcome.”

Mr. Hafetz argued that Mr. Finnerty wasn’t told he was doing anything wrong by the NYSE despite a number of alerts that come up in the computer system to indicate problems in trading.

He questioned the “exception reports” that prosecutors relied upon in their case to identify improper positioning of trades.

Mr. Hafetz said the exception reports don’t identify how many intervening trades occurred between the two sides of the questioned transactions made on Mr. Finnerty’s watch, saying it could be as many as eight to 10 in 14 seconds.

Given the speed of activity on the NYSE, it could be difficult to track sides in trades, and mistakes could be made, he said.

He also questioned the government’s decision to call certain clerks to testify against Mr. Finnerty, saying prosecutors failed to call three clerks who worked with him during the majority of time the questioned trades occurred. One clerk who testified only worked with Mr. Finnerty for a day, while another only worked with him briefly.

Prosecutors argued that Mr. Finnerty’s improper trading was so blatant that the clerk who worked with him for a day told his boss that he was “uncomfortable” and didn’t want to do it again.

Fleet Specialist, now known as Banc of America Specialist Inc., is a unit of Bank of America Corp.