Oil giant Exxon Mobil Corp. has regained the top of the Fortune 500 list of the biggest U.S. publicly traded companies, despite the sharp fall in oil prices.

The list, which was released Sunday, pegs Exxon Mobil's 2008 revenue at about 442.85 billion U.S. dollars, with profits of 11.4 billion.

The Fortune ranking is based on revenues, and three of the top 4 companies on the list came from the oil industry as U.S. largest firms suffered their worst slump ever.

Wal-Mart, which had held the top spot for six of the last seven years, slipped to No. 2, with 405.6 billion dollars in revenue.

Rounding out the top 10 on the list were Chevron Corp; ConocoPhillips; General Electric; General Motor; Ford Motor; AT&T; Hewlett-Packard and Velero Energy.

"Last year was the worst economic performance in the 55-year history of the Fortune 500 list of America's biggest 500 companies," Fortune said in a statement.

The magazine said that earnings dropped 84.7 percent from the previous year, from 645 billion dollars to 98.9 billion dollars, marking the largest one-year decline ever.

President Barack Obama also warned that the risks remain real and significant for the U.S. economy.

"While there have been some encouraging signs that our economy may be stabilizing, the risks remain real and significant," said Obama in an interview.

"There is substantial work left to be done - not only to repair the damage immediately, but to ensure that we emerge from this recession with an economic model that leads to durable, sustainable growth," he added.