The SEC charged Cuban, 50, with selling shares of an Internet search engine company in 2004 based on inside information from the firm's chief executive. The lawsuit asks that Cuban return the $750,000, plus pay interest and an unspecified fine.

"Mr. Cuban was entrusted with nonpublic information after he had promised to keep the information confidential," said Scott Friestad, deputy director of the SEC's division of enforcement. "He sold all of his shares prior to that information being publicly available in a way that allowed him to profit from his access to that public information."

After being told by the chief executive of the search site Mamma.com that his 6.3 percent ownership stake would be diluted by a discounted public offering, Cuban "became very upset and angry" and complained he now couldn't sell the stock, the lawsuit says.

But hours later, the SEC says, Cuban placed orders to sell all of his 600,000 shares at prices ranging from $13.2937 to $13.499. After the discounted offering was announced two days later, the stock began a downward spiral from which it has not recovered. Shares of the company, now known as Copernic Inc., closed yesterday at 28 cents.

Cuban, who had been in New York Sunday for a game between the Mavericks and Knicks, didn't respond to an e-mail seeking comment. He posted a statement on his blog from his attorney, Ralph Ferrara, who called the lawsuit "a gross abuse of prosecutorial discretion.

"The government's claims are false and they will be proven to be so," the statement said.

Cuban, the 161st richest American, according to Forbes, with a net worth of $2.6 billion, is the best known and most controversial of NBA owners, having paid more than $1 million in fines to the league. Neither NBA commissioner David Stern nor league president Joel Litvin would comment on the lawsuit.