Diesel and gasoline prices continued to drop for the 10th straight week, and European economy concerns are still affecting oil prices.

The average U.S. diesel price fell 5.2 cents this week to $3.729 per gallon, more than 22 cents below diesel fuel prices from one year ago, according to the Energy Information Administration's weekly fuel update. Diesel prices are at their lowest since October 2011.

Prices still hovered near $4 per gallon in California, the most expensive area of the country, at $3.966. The lowest prices were in the Gulf Coast region at 3.654.

Gasoline prices fell by 4 cents to $3.572 per gallon, down more than 14 cents from a year ago. All regions saw price decreases except the Midwest, which experienced a 2-cent increase per gallon.

Monday, U.S. West Texas Intermediate crude fell by 76 cents to settle at $83.27 per barrel. Brent crude, which helps set the price for much of the oil imported into the U.S., fell $1.56 to $96.05 per barrel in London.

Investors worried Spain could default on its debts as yields on Spain's bonds soared, causing oil prices to decline. Also, a weekend election in Greece resulted in victory for a party in favor a bailout of the country's failed economy.