A 28-year-old District of Columbia man who tried to flee after allegedly selling LSD to an undercover Fairfax County policeman was shot accidentally by another investigator yesterday afternoon as he was being pursued over a fence, Fairfax police said.

Mario R. Rivera of 120 Webster St. NE was undergoing surgery in Fairfax Hospital late yesterday for a gunshot wound to the chest, according to hospital officials and police.

He was listed in critical condition.

The incident occurred about 1 p.m. after an undercover drug purchase in an unmarked black Dodge Daytona parked at the Yorktowne Shopping Center on Arlington Boulevard (Rte. 50) and Gallows Road, according to Fairfax police spokesman Warren Carmichael.

He said the investigator, whom he declined to name, had just bought 900 "hits" of LSD, with an estimated street value of $4,500, and had signaled other officers to help make an arrest.

Rivera fled as a marked Fairfax police car arrived and as officers converged on the scene, Carmichael said. He said Rivera climbed a 6-foot wire fence that separates the parking lot from construction work on Arlington Boulevard.

Dennis DeBord, an undercover vice and narcotics investigator, followed Rivera over the fence, his .38 caliber service revolver drawn, Carmichael said. He said DeBord lost his footing at the top of the fence, fell and hit his head on the ground as the two landed simultaneously.

"They both fell on the pavement about a 10-foot drop ," Carmichael said. "As they fell, the investigator's weapon discharged accidentally."

Peter Sherwood, a uniformed officer waiting as part of the "surprise" arrest team, followed DeBord over the fence and suffered minor cuts, Carmichael said.

DeBord, 34, was treated for cuts and bruises to the head at Fairfax Hospital and then released.

Sherwood, 25, also was treated and released.

Carmichael said DeBord, a 10-year veteran of the police department, acted properly in drawing his revolver. "That is standard procedure in the case of a drug buy bust," he said.

DeBord has been placed on administrative leave, the standard procedure whenever an officer's gun discharges, while an internal investigation of the incident continues, Carmichael said.

The drug buy, he said, was part of an investigation of Rivera as an alleged supplier of LSD.

No charges had been filed against Rivera late yesterday, Carmichael said.