Activist investor William Ackman said on Wednesday that his firm recently bought 150 million shares of Citigroup Inc , but didn't provide his reason or an investment thesis for the purchase.

At the end of his speech at the 15th annual Ira Sohn investment research conference, Ackman said, And by the way, we bought about 150 million shares of Citigroup, but I don't have time to talk about it.

Ackman is the managing member of Pershing Square Capital Management.

(Reporting by Herbert Lash; Editing by Gary Hill)