Warren Buffett's Firm Buys Big Stake in Exxon Mobil

Stake worth $3.45 billion

Warren Buffett's Berkshire Hathaway bought a $3.45-billion stake in Exxon Mobil in the third quarter, the company's largest investment since it invested $10.7 billion in IBM two years ago, according to a regulatory filing late Thursday.

The Securities and Exchange Commission filing shows that Berkshire Hathaway held about 40 million Exxon Mobil shares as of Sept. 30.

The company, however, appears to have begun amassing its shares during the second quarter. A separate amended filing Thursday showed that it had bought the bulk of the Exxon Mobil shares, about $2.82 billion worth, in the three-month period ending June 30.