The malaise on Wall Street simply will not lift.

Despite cheaper oil, a stronger dollar and looser credit, stock investors on Wednesday could not shake their fears about the consequences of an economy that many believe is already in a recession. Worries that slowing growth will pummel the corporate sector sent the Dow Jones industrials down more than 500 points for the day.

The broader Standard & Poor’s 500-stock index fell 6.1 percent, a decline that in any other financial environment would be considered extraordinary. Instead, it counted only as the worst loss since last Wednesday, when the S.& P. plunged 9 percent.

The slump overshadowed a milestone for oil prices, which tumbled to $66.75 a barrel, a 16-month low, and strong gains in the American dollar against foreign currencies.

In the stock market, the bloodletting began after corporate bellwethers from a range of sectors across the economy  including Boeing, Merck and Wachovia  warned of a bleak outlook for the remainder of the year as they reported disappointing earnings for last quarter.