Firefighters responded Wednesday to a report of a vegetation fire off Highway 1 near Gaviota.
Crews were dispatched shortly before noon to the blaze, dubbed the Cruces Fire, which reportedly was 2 to 3 miles west of Highway 101.
A second alarm was called at about 12:10 p.m.
The fire was burning north of Highway 1 and west of Highway 101.
As of 1 p.m., the fire had blackened about 150 acres, with the potential to grow to 500 to 1,000 acres, according to Capt. Dave Zaniboni of the Santa Barbara County Fire Department.
Evacuations were in progress in the 7200 Block San Julian Road, and along Highway 1 between Highway 101 and El Jaro Creek.
The fire was reported to be burning in medium to heavy vegetation, with a moderate to fast rate of spread.
Firefighters were expressing concern that the blaze could jump Highway 1 and move south toward Hollister Ranch. An alert was issued to residents on that area, but no evacuations have been ordered there, Zaniboni said.
Helicopters and air tankers were ordered to assist in fighting the fire.
There are some structures in the area, but it was not immediately known if any were threatened.
Officials were using the 9-1-1 system to alert residents in the area.
The CHP shut down Highway 1 between Highway 246 and Highway 101.