Citadel Securities’ Stock Trading Push Sets Wall Street on Edge

Jamie Dimon’s letter to JPMorgan Chase & Co. shareholders published this month runs to about 25,000 words. For Wall Street, two in particular stand out: “Citadel Securities.”

The rare callout of the market maker by the CEO of America’s biggest bank in his annual missive — in a section on the lender’s fierce competition — comes as Citadel Securities makes a bold new push into territory dominated by JPMorgan and its peers: Handling equity block trades for the likes of hedge funds, asset managers, and pensions.

It’s the opposite of the small-size, high-volume stock business on which ...