The latest slide came as OPEC said its production rose to a three-year high. China also devalued its currency, suggesting economic growth there was softer and could cause lower crude demand.
U.S. crude has been declining since reaching a high this year of $61.43 on June 10.
Crude is under pressure on several fronts. Big increases in production in the U.S. and Canada, along with sizable gains in Iraq and elsewhere, have helped increase supplies. Saudi Arabia and other OPEC nations kept pumping crude at high levels and Iranian oil could soon return to the market after being kept off by sanctions.
Meanwhile, worldwide demand is not as strong as expected because China's growth has cooled and other economies have become more energy efficient.
While drivers, shippers and airlines are enjoying the lower fuel prices spurred by crude's slump, the oil industry is responding to lower profits with sharp cuts in spending and employment.
In other futures energy trading, Brent crude, a benchmark for international oils used by many U.S. refineries, declined $1.23, or 2.4 percent, to $49.18 a barrel in London. Wholesale gasoline closed unchanged at $1.694 a gallon, while heating oil fell 2.9 cents to close at $1.563 a gallon. Natural gas rose 0.2 cents to close at $2.844 per 1,000 cubic feet.