The Dow Jones industrial average is closing more than 300 points lower, its worst loss in two months.
Traders were discouraged Friday by another slump in the price of oil and more signs of weakness in China.
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The Dow dropped 315 points, or 1.8 percent, to 17,280. The Dow had its first losing week since October.
The Standard & Poor's 500 fell 33 points, or 1.6 percent, to 2,002. The Nasdaq slid 54 points, or 1.2 percent, to 4,653.
Oil prices sank again after the International Energy Agency said global demand will grow less than previously forecast next year.
Crude tumbled $2.14, or 3.6 percent, to $57.81 a barrel. It's down 47 percent since June.
Bond prices rose. The yield on the 10-year Treasury note fell to 2.09 percent.