The brokerage said its clients made an average of 113,000 online trades in August, down from 135,000 in July and 137,000 in June. The decline led the company to predict that current-quarter profits will be at the low end of its previous guidance for earnings of 10 cents to 22 cents a share.

Analysts surveyed by Thompson First Call were forecasting earnings of 12 cents a share in the quarter.

In August, a month that was characterized by listless trading amid signs of an economic slowdown and terror jitters ahead of the Republican National Convention, Ameritrade said it opened 20,000 new accounts and closed 10,000 accounts. Average client margin balances were $3.2 billion in the month, while client assets were $67.4 billion.

The stock was recently down 15 cents, or 1.3%, to $11.60.