A brush fire scorched five acres near Castaic Lake and prompted the closure Tuesday morning of the southbound 5 Freeway.
The fire, which started at about 10:39 a.m., was first spotted burning along the freeway and upper crossover, said John Wagner, spokesman for the U.S. Forest Service.
Flames ran uphill south of Paradise Ranch, west of Castaic Lake and toward radio towers, authorities said.
A Los Angeles County Fire Department helicopter dropped off 11 firefighters to assist in fighting the fire, which was held at five acres.
At 11:22 a.m., the California Highway Patrol issued a SigAlert, shutting down all southbound lanes. By 12:26 p.m., two lanes had been reopened, but fire mop-up efforts continued.
The remaining lanes will be closed until further notice.