The Farifax County prosecutor said yesterday he expects to seek a hearing at which the County Juvenile Court will determine whether 15-year-old Larry Viscidi will be tried as an adult in the slaying of his 12-year-old brother Billy.

Commonwealth's Attorney Robert F. Horan Jr., who refused to refer to Larry Viscidi by name because of confidentiality requirements of Virginia Juvenile law, said he will request a so-called transfer hearing if "probable cause" linking the suspect to the slaying is established at a preliminary hearing scheduled for Oct. 10.

Larry Viscidi was arrested Sept. 13 and charged in Juvenile Court in the death of his brother Billy, who died on July 25 from a blow to the back of the head. Billy, who was first thought to be runaway or kidnap victim, was found Aug. 12 buried in the backyard of his home at 503 Princeton Ter. in Vienna.

Larry was released from police custody on Sept. 18 in the custody of his parents, and is now living at home.

Horan said yesterday that his request for transfer hearing will require that a Juvenile Court probation officer make an investigation of the juvenile's suspect's social history.

Vincent M. Picciano, director of juvenile court services, said yesterday that such an investigation will look into the boy's relationship with his family, his schoolmates and neighbors.

"What they try to obtain is a broad picture of the juvenile - what his personality is and how does he fit in the community. The investigation also studies what community alternatives there are for treating the juvenile," Picciano said.

Using the Juvenile Court investigation and information gathered by police in their investigation of Billy's death, Horan said he will make his final decision about whether to ask in the transfer hearing that "the suspect be certified for trial as an adult.

Under Virginia juvenile law, a youth can only be certified for trial as an adult if he is 15 years old or older when the offense was committed, if probable cause is found that he committed the crime, if the court decides he is "not amenable to treatment . . . through available facilities," if he is not mentally retarded or criminally insane and if "the interests of the community require that the child be placed under legal restraint or discipline."

Deputy Commonwealth's Attorney Steven A. Merril has said it is always difficult to persuade a Juvenile Court judge to certify an accused juvenile as an adult in any case.