(Reuters) - Warren Buffett's Berkshire Hathaway Inc BRKa.N on Wednesday said it boosted its stake in Amazon.com Inc AMZN.O by 11% during the second quarter, increasing its bet on the powerful online retailer even as stocks traded near record highs.

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Berkshire said it ended June with 537,300 Amazon shares worth about $1.02 billion, up from 483,300 shares three months earlier.

It made the disclosure in a regulatory filing detailing the $208.1 billion of U.S.-listed stocks it owned as of June 30.

Buffett, the world’s fourth-richest person according to Forbes magazine, also drew attention in the quarter from fellow billionaire William Ackman, whose Pershing Square Capital Management revealed a $749 million Berkshire stake.

Berkshire’s stock price is more than 12% below its record high set in October.

The Omaha, Nebraska-based conglomerate also owns more than 90 businesses such as the BNSF railroad, Geico auto insurance and Dairy Queen ice cream.

Berkshire had disclosed earlier this month that it sold more stocks than it bought during the quarter, suggesting possible concern among Buffett and his portfolio managers Todd Combs and Ted Weschler about valuations.

Wednesday’s filing includes investments by all three without saying who bought and sold what, though Buffett has said Combs or Weschler made the initial Amazon investment.

It was unclear whether Berkshire did any buying or selling on other exchanges during the quarter. Buffett’s assistant did not immediately respond to a request for comment.

The biggest investment to exit Berkshire’s portfolio was a roughly $1.7 billion stake in USG, a wallboard maker acquired by Germany’s Knauf KG in April.

Berkshire had agreed to sell its roughly 31 percent stake in USG, a nearly two-decade old investment that Buffett had labeled disappointing, as part of that merger.

The net selling contributed to Berkshire’s ending June with a record $122.4 billion of cash and equivalents.

Berkshire invested $10 billion of that cash this month in Occidental Petroleum Corp OXY.N, which completed its takeover of Anadarko Petroleum Corp.

Berkshire's biggest stock market commitment remains in financial services, where it is the largest shareholder in American Express Co AXP.N, Bank of America Corp BAC.N and Wells Fargo & Co WFC.N and has a significant stake in JPMorgan Chase & Co JPM.N, where Combs is a director.

Other changes in Berkshire's portfolio including a small increase in U.S. Bancorp USB.N and decrease in Charter Communications Inc CHTR.O.