U.S. stocks are closing lower, extending losses from last week.
The price of crude oil continued to drop Monday, and Russia's ruble plunged to a record low against the dollar.
The Dow Jones industrial average fell 100 points, or 0.6 percent, to 17,180.
The Standard & Poor's 500 index fell 12 points, or 0.6 percent, to 1,989. The Nasdaq composite lost 48 points, or 1 percent, to 4,605.
Russia's ruble plunged 13 percent against the dollar. The falling price of oil, which is the chief source of Russian exports and tax revenue, has weighed heavily on the currency.
The price of U.S. benchmark crude oil sank $1.90, or 3.3 percent, to $55.91 a barrel. Oil has fallen by nearly half since June.