Hedge fund manager Barry Rosenstein has likely made a killing on Microsoft

Barry Rosenstein's hedge fund, Jana Partners, took a big position in Microsoft last quarter, according to its latest 13F filing.

And it likely made a killing.

The stock was trading at $47.23 at the beginning of the third quarter, and fell to as low as $40.47 during the period.

It closed the quarter at $44.26, and was trading on Monday at $53.76.

Jana's position in Microsoft had previously been reported, but the size of the position wasn't disclosed until today.

At the end of the quarter the position was valued at $340 million. That makes it the fund's ninth biggest holding.

Jana's biggest new holding, at $413 million, was Baxter International. That stock is down about 7% since mid-August.

13Fs are documents hedge funds must file with the SEC disclosing their long positions within 45 days of the prior quarter's end.