Mark Cuban is telling jurors in federal court about calls and e-mails he sent to the CEO of an Internet company shortly before he sold all his shares in the company.

The billionaire owner of the Dallas Mavericks began testifying Thursday in the government's insider-trading lawsuit. The Securities and Exchange Commission accuses him of using confidential inside information to dump his shares before the company announced news that sent the stock down.

Cuban is jousting at times with the SEC lawyer questioning him, but he's also been smiling and making a few jokes.

The case is a civil lawsuit. The SEC wants Cuban to pay a fine and give up $750,000 in losses he avoided by selling the stock, but he's not charged with a crime. Cuban denies wrongdoing.