An off-duty Prince George's County policeman shot and killed a Louisiana man yesterday after a traffic dispute on a Capital Beltway exit ramp at Rockville Pike in Bethesda, Montgomery County police said.

According to police, Prince George's County police Sgt. Benny G. Gilbreath was driving his own van and was on the westbound ramp toward Rockville Pike after exiting the Beltway when a pickup truck driven by Richard F. Calvird tapped the bumper of Gilbreath's van.

Gilbreath told police that after the first contact, he was slowing his van and pulling it to the right side of the road when Calvird's vehicle struck the rear of his van. The pickup's hood was wrinkled, but little damage was done to the policeman's van.

Gilbreath told investigators that Calvird got out of his vehicle and approached him, brandishing a large knife and cursing, police said. Montgomery County police spokesman Sgt. Harry Geehreng described the weapon as "a bowie or a buck knife, the type used by hunters."

Geehreng declined to say whether Gilbreath had informed Calvird that he was a police officer, or whether Calvird was warned that he would be shot if he continued walking toward Gilbreath. But, Geehreng said, "several witnesses told us that there was little else that the policeman could do." Gilbreath fired his .38-caliber service revolver more than once, hitting Calvird in the left side of the chest. Police would not say how many times the gun was fired. Police closed the Beltway exit for more than two hours, causing a traffic backup.

Police said Calvird, 38, was from Kenner, La., and, based on papers in his vehicle, believe he was working at a construction site in Prince George's County. His body was taken to the state medical examiner's office in Baltimore for an autopsy.

Gilbreath is a 16-year veteran of the Prince George's County police force and a member of the Special Operations Division's tactical team. Prince George's County police must carry guns at all times when in the county, and are permitted to carry them in other Maryland jurisdictions. No charges have been filed and Gilbreath was placed on routine administrative leave pending the results of a Montgomery County investigation.