Twice in the past week, motorists driving nice, late-model cars have sped off like practiced bank robbers after filling up at the Mobil station in Fairfax where K.C. Nawras works as a cashier.

Nawras keeps the receipts, one for $20, the other for $27.32, taped to the wall above the cash register, on the off chance the gasoline thieves are so brazen as to come back for another refill on the sly.

"I know the color of their cars; I'll recognize them," vowed Nawras, who has on occasion given chase on foot and caught up with surprised gas thieves at stoplights.

Soaring gas prices in the run-up to Memorial Day, which usually has the most expensive gas of the year, have contributed to a notable increase in drive-offs at gas stations and convenience stores. Although no statistics have been collected, industry officials say the increase is striking in contrast with a pronounced decrease after many states passed laws defining gas theft as a prosecutable crime.

Today's thieves are as likely to be driving Mercedeses and SUVs as rattletraps, suggesting they are not motivated by economic need.

"It's frustration and misdirected anger," said Jeff Lenard, a spokesman for the National Association of Convenience Stores. "A lot of people are very upset by high prices and feel somebody needs to be held accountable. Unfortunately, the people they're taking it out on are probably the least responsible for high prices."

Even with gasoline prices north of $2 a gallon, the retailer usually gets only 2 or 3 cents per gallon, industry officials say. The rest is eaten up in the costs of crude oil, refining and transportation and credit card fees.

"Retailers make more off a 12-ounce cup of coffee than they can on a fill up," said Michael O'Connor, president of the Virginia Petroleum, Convenience and Grocery Association.

That is one reason so many convenience stores sell gasoline, and so many gasoline stations sell the sundries of convenience stores. The gas is, in effect, a lure to get customers inside the store to buy something much more profitable for the retailer.

Nationwide, 106,000 of the country's 130,600 convenience stores sell gas, according to the industry association. In Virginia, almost 80 percent of the convenience stores sell gas, compared with 62 in Maryland and 38 percent in the District.

Convenience stores are reluctant to ask customers to prepay for their gas, because the customer is less likely to make a second trip into the store and buy a high-profit cup of coffee or candy bar.

"You tend to lose customers if you mandate prepay," Lenard said. "It takes the convenience out of convenience store. And while you've made pennies at the pump, you can make dollars at the store."

The economics mean that even rare occasions of gasoline theft can eat into a large chunk of a retailer's profit.

O'Connor said he believes drive-offs have increased by 15 percent to 20 percent in the past month. Before Virginia passed a law in 2000 criminalizing gasoline theft, the incidence was about six-tenths of one percent. Since the law went into effect, that rate had decreased substantially to four-tenths of one percent. O'Connor said it might now be returning to its pre-law level.

Lenard said individual retailers have told him that gas theft is costing them $800 or more a month.

"I've heard it's gone from once or twice a week to once or twice a day," he said. "That's enough to wipe out your profits. People come in with empty tanks. With the prices high, we're talking about $60 thefts."

Some retailers, such as Soon J Kim, owner of a Shell gas station and convenience mart off I-66, said that gasoline theft has become so prevalent that she does not even report it to police anymore.

"It's happened four times or five times, maybe seven times, in the past week," said Fatima Zahara, who works the cash register at Circle Sunoco in Fairfax. "I've lost count. They go to the farthest pumps, and when they're done, they don't put the nozzle back. They just drive away."

One day last week, Circle Sunoco was hit twice. One motorist left without paying for a $25 fill up; another rang up $27 of gas and fled.

"If we see the tag number, we call police," she said. "Otherwise, it comes out of my pocket."

The problem is particularly acute off interstate highways. At an Exxon station near I-66, cashier Krishan Chander said some gas thieves pull in with license plates that are fake or folded over so they cannot be read.

"They're expensive cars," he said, noting that on Thursday a black Nissan sports car left without paying for a $38.65 fill up. The station has posted signs on the farthest island of pumps asking customers to pay before pumping, but they do not intend to institute the same policy at all eight pumps.

"We're being cautious," Chander said. "If you ask people to pre-pay, some don't have time and become annoyed. We try to make a balance."