Fairfax County police believe two men are responsible for a string of nine armed robberies of fast-food restaurants in the county since late August.

The suspects, armed with a "long-barreled, large-caliber revolver" and a "large-caliber automatic pistol," according to police, have typically hit the chain restaurants -- Arthur Treacher's Fish & Chips, Pizza Hut, Hardee's Hamburgers and Gino's -- near closing time.

In each of the robberies, police said, the suspects have entered the restaurant with guns drawn, forced the manager to open the safe, taken money and jewelry from customers and employes, and then fled, apparently on foot.

Fairfax County police Lt. Edward Stevens said yesterday that police in Arlington County and Alexandria are investigating similar robberies in those jurisdictions, and that the same suspects may be involved. Stevens said there was no geographical pattern to the robberies. "It's a hit-or-miss type of thing," he said.

"Right now, the investigation is a routine hitting of the trenches," Stevens said. "It seems like the robberies go in spurts. We're probably dealing with drugs or drug addicts, so it's a continuing thing, day to day. They hit the restaurants, spend the money, then have to hit again to buy more drugs."

A $1,000 reward for information leading to the arrest and conviction of the suspects has been offered by Fairfax police.

John S. Cockrell, of the Restaurant Association of Metropolitan Washington, said specific numbers of fast-food-chain robberies are not available. "There have been an awful lot of them, though," he said.

Wayne Pierce, manager of a fast-food restaurant police believe was robbed by the suspects, said the men were "unbelievably professional."

"They came in, walking together, almost in step, and one of them said, real quiet and calm, 'This is a holdup,' like gee whiz, we have to rob another place, ho-hum, like this was their regular job. They didn't make any quick moves like on TV, like on Hill Street Blues . . . They moved with precision, didn't say more than two or three sentences and they were out of there in maybe two, two and a half minutes.

"You could tell these guys knew what they were doing. They didn't make you nervous at all."