The biggest insider trading case in Wall Street history widened yet another notch yesterday, as another former employee of Lazard Freres & Company received a subpoena as part of the stock trading investigation centered on Dennis B. Levine, the 33-year-old former merger official.

Lazard Freres, one of the nation's most active advisers in the takeover business, said that Randall D. Cecola, 25, a first-year student at Harvard Business School, who left the firm last year, was under investigation in the case.

Alan Kaufman, a former Assistant United States Attorney who is Mr. Cecola's lawyer, said: ''All that's happened is that Mr. Cecola received a subpoena a few days ago to produce documents. No one in the Government has informed me he's under investigation. Based on my preliminary analysis of a day and a half, I see no basis that he's violated insider trading laws.''

Sources close to Lazard said that the firm had received a subpoena dealing with Mr. Cecola's activities. The firm said it was cooperating with the investigation. These sources said that earlier in the week Mr. Cecola had visited Lazard officials to tell them his side of the story.