The volatile stock market proved to be a tonic for online broker TD Ameritrade ( AMTD), which posted a 67% jump in quarterly profits. In the firm's fiscal third quarter, Ameritrade earned $139.8 million, or 23 cents a share, up from $83.6 million, or 20 cents a share, a year ago. Net revenue more than doubled to $540.3 million. Earnings at the Omaha-based brokerage were fueled by big gains in commission revenue and interest earned on customer margin balances. The broker beat the Thomson Financial consensus estimate of 22 cents a share by a penny. But revenue fell a bit short of the consensus forecast of $543.8 million. Overall, it was a solid quarter for Ameritrade. Many on Wall Street have begun worrying that the up-and-down stock market would drive some retail investors to the sidelines. But for now at least, Ameritrade appears to be feasting on the volatility. Ameritrade, meanwhile, offered new earning guidance for its upcoming fiscal year, which begins in October. The firm says it expects to earn between 99 cents a share to $1.21 a share in the coming year. Currently, according to Thomson Financial, the analyst consensus forecast has Ameritrade earning $1.20 a share in 2007. In the quarter, revenue from trading commissions and fees rose 88% to $213 million. An even bigger boon to Ameritrade in the quarter was the doubling in net interest revenue to $199 million. Net interest revenue is the income the broker earns from charging interest to its customers who trade on margin, or with borrowed money. The Federal Reserve's decision to continue hiking short-term rates has meant more money for the coffers of Ameritrade and other brokers with customers who trade on margin. Shares of Ameritrade closed Monday at $13.64. In premarket trading, the stock was trading higher, rising 37 cents, or 2.7%, to $14.01.