Berkshire Hathaway (NYSE:BRK.A), Warren Buffett’s investment company, owned 40.1 million shares of Exxon Mobil (NYSE:XOM) on Sept. 30, media reports said, citing regulatory filings.

Berkshire’s stake, which is worth about $3.7 billion at Thursday's closing price and was disclosed in a quarterly regulatory filing submitted Thursday, pushed Exxon’s stock up in extended trading to $93.90, Bloomberg reported.

Irving, Texas-based Exxon, which is the world’s largest oil company by market value, “is undervalued, in [Buffett’s] opinion, and pretty much being ignored by the market,” David Kass, a professor at the University of Maryland’s Robert H. Smith School of Business, told Bloomberg. “He knows the company. He knows it well.”

The quarterly regulatory filing also showed that the Omaha, Neb.-based Berkshire significantly reduced its stake in another energy major, ConocoPhillips (NYSE:COP), by 44 percent in the September quarter, reports said. Berkshire brought down its stake in the company to 13.5 million shares at the end of September, down from 24.2 million shares in the June quarter. The Houston-based energy major’s stock ended up 0.46 percent on Thursday.

Berkshire’s stake in Exxon is the holding company’s largest new stake since it invested in International Business Machines Corp. (NYSE:IBM) in 2011, according to the Bloomberg report.

The quarterly filing provided an update on Berkshire’s $92 billion U.S. stock portfolio, and investments in Exxon and ConocoPhillips were the biggest changes, USA Today reported.