NEW YORK (MarketWatch) -- Warren Buffett is having a bad day Thursday. His Berkshire Hathaway Inc. BRK.B, +0.07% is American Express Co.'s AXP, -1.16% biggest shareholder, according to a FactSet analysis of the most recent regulatory filings. Berkshire owned 151,610,700 shares of the charge card company as of Sept. 30. With AmEx's stock down $5.15, or 6%, in midday trade, after the company said it was losing its exclusivity deal with Costco Wholesale COST, -0.86% , that would mean Berkshire could be losing $780,795,105 on its holdings today. Investors will get an update of Berkshire's holdings soon, as institutional investment managers must report by Feb. 14 what stocks they held as of Dec. 31. Berkshire's Class B stock was down 0.1% in midday trade. It is up 2.8% over the past three months, ahead of the S&P 500's 2.2% gain.