The average price of diesel fell 6.4 cents in the past week to $3.997 a gallon across the U.S., the first time it has been below $4 in nearly two months, according to the U.S. Energy Information Administration.
The third straight weekly decline followed recent declines in the price of crude oil on world markets and forecasts of plentiful oil stocks heading into the summer driving season.
The EIA measured the steepest drop in California, where the average price fell 8.4 cents in the week ending May 23, although the average price in the state of $4.287 was still tops in the country.
The national average price has fallen 12.7 cents, or 3.1 percent, since reaching its 2011 high of $4.124 a gallon the week of May 2.
The price has remained beyond $4 a gallon since the week of April 4.