The fire was burning west of the 5 Freeway.
Campgrounds in the area were evacuated.
The fire was reported shortly before 2 p.m. Saturday.
Firefighters from Ventura County and the Calif. Dept. of Forestry and Fire Protection were working together to battle the blaze.
Authorities said most of the firefighting attack was from the air.
No injuries were reported. No structures were threatened.
A containment estimate was not released.