Exxon Mobil is now the largest US publicly-traded company by market value - roughly a year after losing the title to Apple.
Apple's stock has kept on declining since it released its earnings report on Wednesday.
It dropped 2% on Friday to $441.30 (£279.36) for a market capitalisation of $414.5bn (£262.4bn).
Exxon Mobil, meanwhile, gained 13 cents to $91.48 (£57.91) and has a market capitalisation of $417bn (£264bn).
Apple's earnings results suggested that its phase of fast growth - which is rare for a company of its size - may be coming to an end.
On Thursday, the stock saw its biggest one-day percentage drop since 2008.
Apple's market capitalisation has now fallen by about $250bn (£158bn) - roughly the market value of Google - since hitting a high last September, when the stock traded above $700 (£443).
Apple first surpassed multinational oil and gas firm Exxon in the summer of 2011.
The two companies swapped places that autumn, until Apple surpassed Exxon for good in early 2012 before their latest reversal of fortunes.
"Apple was clearly a momentum stock," said Kim Forrest, a senior equity research analyst at Fort Pitt Capital Group.
"Whenever the numbers behind momentum stocks stop, the momentum players are out and the stock tumbles."
Nevertheless, Apple still managed to ship a record 47.8 million iPhones in the December quarter, up 29% from a year earlier.
However, that lagged behind the 50 million phones that analysts on average had projected.
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