At issue in the S.E.C.’s lawsuit is Mr. Cuban’s sale in June 2004 of shares in Mamma.com, a small Internet search engine based in Canada, whose corporate name is now Copernic.

Mr. Cuban had purchased 600,000 shares, or a 6.3 percent stake, just three months earlier as the stock was soaring. The share price tripled over a two-day period in early March on volume that totaled more than 12 times the number of outstanding shares. That prompted an S.E.C. investigation that ended without charges being filed.

Scott W. Friestad, the S.E.C.’s deputy director of enforcement, said the investigation of Mr. Cuban’s trading began in early 2007, but declined to say what had set off the inquiry.

On June 28, 2004, Mr. Cuban called Mamma.com’s chief executive after receiving an e-mail message from the executive, who told him of a planned stock offering and asked if he would like to invest. Such offerings often depress share prices, at least temporarily.

According to the complaint, Mr. Cuban was told the information was confidential.

After the conversation, the chief executive wrote to the company’s chairman in an e-mail message: “As anticipated, he initially ‘flew off the handle’ and said he would sell his shares (recognizing that he was not able to do anything until we announce the equity).”

But within minutes of the call Mr. Cuban began selling his shares, and completed the sales on June 29, according to the lawsuit, fetching an average of $13.24 a share. The next day, after the offering was announced, Mamma.com stock opened at $11.89, sparing him a $750,000 loss. By July 8, the shares had plummeted to $8. On Monday, the stock closed at 28 cents.

“Mamma.com entrusted Mr. Cuban with nonpublic information after he promised to keep the information confidential,” Mr. Friestad said. “Less than four hours later, Mr. Cuban betrayed that trust by placing an order to sell all of his shares. It is fundamentally unfair for someone to use access to nonpublic information to improperly gain an edge on the market.”