A 27-year-old Silver Spring cab driver was shot and seriously wounded early yesterday when a Montgomery County police officer fired a shotgun at a robbery suspect in the cab's back seat, police reported.

The driver, Roger Caines, was in the line of fire when the armed ducked out of the way, according to police.

Caines, whose address was listed as Irving Street NW, suffered at least four shotgun pellet wounds to the head, police said, and was reported in serious condition last night at Washington Adventist Hospital.

The officer, Thomas Sittner, was reassigned to duty at a station house pending a probe by the police force's internal investigations unit. The robbery suspect was apprehended and charged with kidnaping Caines as well as with two holdups.

Police said the incident began when a man who had checked into the Sheraton Inn, 8727 Colesville Rd., Sunday night placed a call from the hotel lobby at 1:22 a.m. yesterday to the Silver Spring Cab Co. and asked for a taxi to take him to the Greyhound Bus termnal in Washington.

The man then produced a gun and robbed the night auditors at the hotel's front desk of about $400, police said, then boarded Caines' cab, which had been dispatched in response to his call.

The auditors told police the man was in the taxi for about three minutes before it pulled away. They said they called the cab company to get the vehicle's number and relayed the information to the police.

A brief chase ensued; Montgomery police joined by D. C. police, pursued the cab as it headed south into Washington on Piney Branch Road and headed it off between Blain Road NW and the B & O Railroad crossing, police said.

At this point, according to police, Sittner, who was one of the first officers on the scene, walked to the left rear bumper of the taxi carrying a shotgun and ordered the occupants of the vehicle to get out.

When the man in the rear seat pointed a handgun at Sittner, the officer fired his shotgun at the cab's rear window, but hitting Caines, police said.

The robbery suspect, Benjamin E. Tomlin, 18, of Los Angeles, was jailed under $50,000 bond in the District on charges of kidnaping Caines and of robbing a Fairfax County cab driver on Key Bridge Sunday. He has also been charged by Montgomery County police with armed robbery of the Sheraton Inn.

Montgomery County police spokeswoman Nancy Moses said Sittner, 26, a four-year veteran of the force, was following "standard operating procedure" in approaching the cab with his shotgun since the officer was in pursuit of an armed felony suspect. She said the men in the cab failed to respond to Sittner's orders to come out of the car and that the man in the back seat pointed a pistol at him.

Moses said an interstate agreement permits police to pursue fleeing suspects across state lines.

John Dimig, Caines' employer at the Silver Spring Cab Co., said Caines worked every Friday, Saturday and Sunday night in the 12-hour shift known as "the weekend special."

Dimig, who described Caines, a bachelor, as friendly, quiet, and easygoing, said Caines had worked on weekends for the company for the past five years and was employed during the week as a musician.

Caines had responded to about 30 calls before his stop at the Sheraton Inn early yesterday and had less than three hours left on his shift, Dimig said.

Cab drivers who worked with Caines reacted to his shooting with a mixture of apprehension and resignation. They inquired about his condition as they straggled into the dispatching room for the Silver Spring Cab Co. They traded stories about hold ups they had experienced or had heard about.

"It makes you a little shaky," said part-time cabbie Jim Schroader, "but there's not much you can do. You can't screen the people who come into your cab . . . You can stay at home and not drive and you wouldn't have to put up with all of this. But then you'd have to starve."