Four water-dropping helicopters and about 250 firefighters were on scene battling the blaze. Two air-tanker planes were ordered to assist in the fight.
Ventura County Fire Dept. Spokesperson Bill Nash said the fire was not wind-driven. He said it was fuel-driven and following the topography of the land, and was burning uphill. About 50 homes were threatened.
The California Highway Patrol closed traffic in the area of Loop Dr. except for residents and emergency equipment.
West Loop and East Loop drives have been closed to everyone except to emergency personnel. Mesa Road has been closed to the public, except for residents.
Nash said homeowners were benefited by observing the county's law requiring a 100-foot defensible-space clearing around residences.
A firefighter reportedly sustained a minor ankle injury.
Voluntary evacuations were ordered.
The fire was reported at about 3:15 p.m.
The cause of the fire was under investigation.