SAN FRANCISCO — A few years ago, drums of used french fry grease were only of interest to a small network of underground biofuel brewers, who would use the slimy oil to power their souped-up antique Mercedes.

SAN FRANCISCO — A few years ago, drums of used french fry grease were only of interest to a small network of underground biofuel brewers, who would use the slimy oil to power their souped-up antique Mercedes.

Now, restaurants from Berkeley, Calif. to Sedgwick, Kan. are reporting thefts of old cooking oil worth thousands of dollars by rustlers who are refining it into barrels of biofuel in backyard stills.

"It's like a war zone going on right now over grease," said David Levenson, who owns a grease hauling business in San Francisco's Mission District. "We're seeing more and more people stealing grease because it lets them stay away from the pump, but it's hurting our bottom line."

Levenson, who converted the engine in his '83 Mercedes to run on straight canola oil, has built up contracts to collect the liquid leftovers from 400 restaurants in the last two years.

Last week when his pump truck arrived at Thee Parkside, a dive bar known for its chili-cheese fries, his driver found someone had already helped himself to their barrel of yellow oil.

Grease is transformed into fuel through a chemical process called transesterification, which removes glycerine and adds methanol to the oil, leaving a thinner product that can power a diesel engine. Biodiesel can also be blended with petroleum diesel, and blends of the alternative fuel are now sold at 1,400 gas stations across the country.

But as the price of diesel shoots up, so, too, does the value of grease.

In the last three years, the price of soybean oil — the main feedstock for biodiesel made in the United States — has tripled. Last week, a gallon of crude soybean oil fetched 66 cents on the open market, according to the National Biodiesel Board.

Those kinds of numbers have encouraged biofuel enthusiasts to plunder restaurants' greasy waste, and have even spurred the City of San Francisco to get into the grease-trap cleaning business.

"Restaurants and staff are no longer looking at this material as trash, they're looking at is as something that's about to go into city vehicles," said Karri Ving, who runs the city's new waste cooking oil collection program. "Unless you lock down every trash can, thefts are going to happen."

Drivers for Blue Sky Bio-Fuels, a grease hauler that also manufactures biodiesel for San Francisco's municipal program, often find the 300-gallon dumpster they store outside the Oakland Coliseum nearly dry, despite the dozens of concessions stands that regularly dump their oil there. Losses at that one site alone have cost the company $3,700 in foregone oil revenues in the last year, said Wesley Caddell, the Oakland firm's business developer.

In Kansas, Healy Biodiesel reports thousands of dollars in losses from used cooking oil heists from restaurants near Sedgwick, about 20 miles north of Wichita.

Standard Biodiesel in Seattle recently started working with police to try to catch the fly-by-night home-brewers who are pilfering up to 30,000 gallons of the oil they collect from restaurants every month.

Company officials say oil rustlers typically siphon their supplies into drums of their own, which they take to backyard gins to be brewed for personal use.

As more customers seek alternatives to petroleum-based fuels, biodiesel production has grown from the grassroots to become a multimillion dollar industry.

The National Biodiesel Board reports that U.S. production of biodiesel reached 500 million gallons last year, up from just 75 million gallons in 2005.

To manufacture the renewable fuel legally, biodiesel producers must register with the U.S. Environmental Protection Agency. Biodiesel consumers must also pay the government taxes to help with road upkeep.