Lake County firefighters are battling a wildland fire just southeast of Lower Lake, in the area near Cantwell Ranch and Spruce Grove roads.
The fire, which started sometime after 2 p.m., is believed to have been started by a vehicle, according to a Lakeshore Fire Department dispatcher.
Firefighters and engines from a number of Lake County fire protection districts have responded.
Cal Fire Capt. Jeff Gahagan said that as of 9 p.m. the fire had burned about 150 acres. He said fire is being held at Morgan Valley Road.
"If we hold it at Morgan Valley Road, it will be approximately 500 acres," Cal Fire Battalion Chief Chris Jurasek said, adding that there wouldn't be any more fuel to burn.
Cal Fire has mobilized two helicopters, five air tankers and one air attack plane to battle the fire, which is about 40 percent contained as of 9 p.m. he said.