A 16-year-old from Miami who repeatedly penetrated computer systems of the Defense Department and the space agency has been sentenced to six months in juvenile detention. The Justice Department said he is the first juvenile hacker to be sentenced to serve time.

The youth, Jonathan James, pleaded guilty in federal court in Miami on Thursday to two counts of juvenile delinquency. Had his crimes been committed by an adult, they would have constituted felonies and drawn a sentence of 10 years or more, said Assistant United States Attorney Richard Boscovich.

Attorney General Janet Reno and the United States attorney, Guy A. Lewis, issued statements signaling a tougher stance toward young computer intruders whose deeds might once have been deemed pranks.

''Breaking into someone else's property, whether it is a robbery or a computer intrusion, is a serious crime,'' Ms. Reno said. ''We take computer intrusion seriously and are working with our law enforcement partners to aggressively fight this problem.''