The blaze broke out at approximately 12:40 p.m. in the 6800 block of Wheeler Canyon Road, according to the Ventura County Fire Department. No structures were threatened.
The blaze was initially estimated to be two acres, but spot fires broke out and spread rapidly to surrounding brush. About 50 acres have burned.
Fire crews requested additional fixed-wing aircraft and helicopter support to fight the blaze.
The cause of the fire was not immediately known.