In a press release today, the company said Lewis is leaving for personal reasons. Lewis signed on as CEO in February 1999, initially sharing the position with Ameritrade founder Joe Ricketts.
Ricketts, who was Ameritrade's CEO from 1981 until two months ago when Lewis was promoted, will again resume responsibility as chief executive.
The company would not comment further on why Lewis resigned. "It's a private matter," spokeswoman Donna Kush said.
Analysts who cover the company said they were surprised by Lewis' resignation and could only speculate on the reasons for his departure.
"Clearly, it's one of three things," said Scott Appleby, senior e-finance analyst with Robertson Stephens. "Either he didn't like the direction the company was going, he didn't feel he had enough say over what was happening, or?he got a better offer."
Before joining Ameritrade as co-CEO in February 1999, Lewis worked in both government and the private sector. From 1982 to 1984, he served as President Reagan's head of technology. In 1989, Lewis co-founded consulting company Seers Technologies.
Last month, Omaha, Neb.-based Ameritrade reported fiscal third-quarter earnings that were almost half of those from the same period a year ago. The company earned $4.6 million, or 3 cents per share, in its third quarter compared with $8.9 million, or 5 cents per share, in the year-earlier period. Revenue increased 91 percent to $149 million from year to year.
Analyst said that the decline in profits was related to an increase in advertising costs and that the company has been performing on track.
Ameritrade shares gained 13 cents today to close at $13.06. The company announced Lewis' departure after the close of regular trading. The shares have traded as high as $31 and as low as $10.50 in the past 52 weeks.