The boy and a 15-year-old male were riding dirt bike motorcycles at the dead end of Drive-In Way around 1:45 p.m. when they collided with each other, Sgt. Robert Quintero said.
Both riders fell, but while the 15-year-old had only a complaint of pain following the crash, the 9-year-old suffered a major head injury.
The boy was taken to a hospital by a parent, and flown to Children's Hospital in Oakland after he was stabilized. He remains in critical condition, Quintero said.
Both riders were wearing helmets and protective gear at the time of the crash and parents were on scene supervising. The collision remains under investigation.