Why TD Ameritrade (AMTD) Stock Is Down Today

NEW YORK (TheStreet) -- TD Ameritrade  (AMTD) fell Friday amid continuing concerns about high-frequency trading.

Prominent figures such as Charles Schwab have recently criticized high-frequency trading, a significant source of the financial services company's revenue. Such criticisms have led to growing concern that regulators could take aim at such practices.

Reuters describes this process as "payment-for-order-flow," in which TD Ameritrade receives payments from professional trading firms in exchange for hundreds of thousands of orders that brokerages send to those firms.

The stock was down 3.9% to $30.77 at 11:36 a.m. on Friday.

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