Dark Pool trading: Useful or dangerous?
15 October 2013 Last updated at 02:23 BST
More than one-third of US stocks are now traded through so-called dark pools.
They allow extra-large, electronic trades to happen anonymously through private venues.
However that anonymity worries some regulators and dark pools may face new rules.
Leisha Chi spoke to Seth Merrin, chief executive of one of the world's biggest dark pools, Liquidnet.