He called himself Phineas Phreak and yesterday, at the age of 14, he became one of the first persons to be dealt with under a 1984 Virginia law designed to discourage computer trespassing.

The Montgomery County youth pleaded "not innocent" -- a plea frequently used in juvenile proceedings to avoid giving someone a criminal record -- to electronically breaking into a computer bulletin board service operated by a Vienna man and transferring part of what was stored there to his own computer.

The youth, who had no criminal record, was sentenced in Fairfax Juvenile and Domestic Relations Court to one year of inactive probation and ordered to pay $300 restitution within six months with his own money. Deputy Commonwealth's Attorney V. Britt Richardson, who prosecuted the case, said the youth has a part-time job.

"This is the first such case that I'm aware of in this office involving computer crimes," Richardson said.

The case involved Allen Knapp, 40, who runs the Washington Networks computer bulletin board service part time out of his home at 8222 McNeil St., Vienna. Knapp said his clients typically pay a $10 fee for a password and the opportunity to exchange data with other owners of home computers.

Knapp told a reporter that on Jan. 5 the youth managed to bypass "my normal security safeguards" and erase a substantial portion of Knapp's storage file. He said the youth was able to transfer "one or more" files to his own computer.

"He then called my answering machine, stating what he had done and making certain demands in exchange for the return of the files in his possession," Knapp said.

Knapp said the youth, who identified himself on the tape as Phineas Phreak, had discovered a programming error that enabled him to gain access to the computer system and used that access "to find out my special password."

According to Knapp, the youth wanted the access to obtain files that he would then exchange with his friends.

Knapp said he called the Virginia State Police and the Chesapeake & Potomac Telephone Co. after hearing the message on his answering machine and that the telephone company was able to trace the call and the youth was arrested.

Knapp said he lost about 180 hours of programming and tracking time, which he called "a lot," and sustained about $300 in monetary damages.

Richardson said the court accepted a plea bargain agreement "and imposed the penalty recommended." He added that the case will be dismissed at the end of the probation if the youth does not violate his probation.