A 25-year-old District man pleaded guilty to drug charges yesterday in a case that was made possible through the undercover activities of a 14-year-old girl.

Arlington authorities said the girl, whose name is being withheld because of her age, met Reginald DeWayne Johnson in a convenience store parking lot during the summer to buy a bag of cocaine. Unbeknownst to Johnson, the girl was equipped with an electronic listening device and was being watched by at least 10 police officers.

Law enforcement officials praised the girl's work yesterday, saying the operation was successful because she carried out her instructions with calm and determination. They noted that it is unusual to work with someone her age.

"She's a brave young girl and has very supportive parents," said Assistant Commonwealth's Attorney Richard E. Trodden. "I asked her why she did it and she said, 'You know, we live here.' "

The girl and her parents were not available for comment yesterday.

The girl's involvement in the case began June 11 as she and a friend headed to the 7-Eleven convenience store at 2525 N. Lee Hwy. for their regular after-school snack of soda and chips, Trodden said.

In the parking lot of the store, a man approached them and asked if they wanted to buy drugs. They declined but he scribbled down his telephone number and electronic beeper number for them anyway, Trodden said.

The girls then headed to the Arlington Police Department to recount the incident. After consultation with their parents, one of the girls decided to help police apprehend the alleged dealer.

The first meeting ended with no results when it became dark and police called off the meeting fearing for the girl's safety, Trodden said. A second rendezvous was scheduled.

About 2 p.m. on June 17, two men arrived at the parking lot in a black van. The girl was waiting with $300 the police had given her to make the purchase, Trodden said.

According to Lt. John Karinshak, head of the police vice control section, police "swooped up" the girl as soon as the deal was completed.

"It just galled her, I guess, that this guy was so obvious," said Karinshak. "It just irritated her that he was so bold."

Johnson, of 816 48th St. NE, pleaded guilty in Arlington Circuit Court to distribution of cocaine to a minor and possession with the intent to distribute PCP. He is being held without bond in the Arlington jail pending sentencing Dec. 4. He faces up to 50 years in prison.

Another man, Prince Allen Pannell, 24, of 820 46th St. NE, has pleaded not guilty to the same charges and is awaiting trial.