But Mr. Parmigiani and others suspect that the P.R. of the Galleon case glosses over risks that insider trading can and does occur regularly at many Wall Street firms. In their view, it has become institutionalized. The flow of information between a firm’s analysts, its traders and its clients — a lucrative heads-up on stock upgrades and downgrades, for instance — can bolster trading profits, brokerage commissions and, ultimately, Wall Street paydays. Those in the know can get rich before the rest of us know what happened.

“Prosecutors say insider trading won’t be tolerated, that this is justice,” Mr. Parmigiani says of the Galleon case. “But they refuse to acknowledge that their widespread net has a very big hole in it.”

What exactly happened at Lehman? Mr. Parmigiani says traders there were routinely advised of changes in analysts’ company ratings before those changes were made public. That way, Lehman could profit on subsequent market moves. Here is how he describes it: First, research officials tipped off the traders; then Lehman’s proprietary trading desk, which cast bets with the firm’s own money, positioned itself accordingly. Lehman salespeople also alerted favored hedge funds. Only later, he says, were ratings changes made public.

Blowing the whistle on any big corporation, as Mr. Parmigiani tried to do in the case of Lehman, is almost always perilous. Shortly after noting the suspicious trading in Amkor, Mr. Parmigiani says, he was fired for not being a team player. He has since been unable to find work on Wall Street.

John Nester, a spokesman for the S.E.C., said it conducted a careful, thorough investigation of Mr. Parmigiani’s allegations.

“Not only did we investigate the circumstances surrounding each of the research reports specifically identified by Mr. Parmigiani, we conducted a broader examination into the trading by Lehman clients in all companies in which there was a material change in the stock price following the issuance of a Lehman research report,” Mr. Nester said. “We also investigated the communications between 46 Lehman employees and 56 Lehman clients, including nearly 100,000 e-mails.”

The S.E.C. also analyzed voluminous trading data and interviewed numerous Lehman employees, he said. “After all that, there simply was not any evidence in this case to support the conclusion that Lehman, its employees or its clients had committed insider trading.”