But the two are now so close that the change of guard is likely to happen soon.
Apple Inc.'s stock gained 5.9 percent to $374.01 on Tuesday, bringing the iPhone and iPad maker's market capitalization to about $347 billion when the market closed.
Exxon Mobil Corp. shares, meanwhile, closed up 2.1 percent at 71.64. That gives the oil company a market cap of $348 billion as of the market's close.
Other big-name corporations, such as Wal-Mart Stores Inc. and General Electric Co., don't even come close. Apple overtook Microsoft, the previous No. 2, just last year.