Prince George's County police charged a 47-year-old Oxon Hill man yesterday with the rape of one of two 14-year-old foster girls living with him.

A police spokesman said the charge was placed in connection with an incident last Thursday when the suspect allegedly raped one of the girls in the presence of the second girl at his home. Later the spokesman said, the man warned both girls that he would kill them if they told about the incident.

On Tuesday, however, the victim told her former foster mother about the assault and police were notified, the spokesman said. When juvenile division officers and a representative of the county social services department visited the home that afternoon, they found the girls alone and took them the Prince George's General Hospital for an examination, the spokesman ssid.

After further investigation, police said, the suspect was charged with first-degree rape and sexual abuse. The suspect, a construction worker, was being held without bond pending a hearing today in the county's District Court.

The spokesman said yesterday's arrest was the fifth of a foster parent in the county since August 1977 on charges involving physical or sexual abuse of a foster-child. Three of the suspects were women and two were men he said.

The spokesman said the recent incidents have promted discussion between the county's social service department and police juvenile officials in an effort to improve the foster parent screening process. The county has about 400 licensed foster homes, the spokesman said.

The victim in the Nixon Hill incident has been assigned to the home recently after being abandoned by her natural mother, the spokesman said.

The girls have been placed in different foster homes since Thursday's incident, the spokesman said.