The fire broke out on Catalina Island, about 35 km off the coast of Los Angeles, shortly after noon and quickly blackened some 500 acres (200 hectares) as crews raced equipment across the water on military hovercraft.
No homes were destroyed but the flames were moving quickly toward Avalon, a town of several thousand people and a popular tourist destination, prompting authorities to evacuate some residents.
Television images showed thick black smoke billowing into the air over the island as firefighting aircraft dropped water and fire-retarding chemicals on the dry brush.
Firefighters were being ferried to the island on boats.
The Catalina blaze broke out as Los Angeles firefighters gained control of an 817-acre (330-hectare) brush fire that scarred the city's Griffith Park and forced the evacuation of a wealthy Hollywood Hills neighborhood.
The Los Angeles Zoo also evacuated visitors and most of its staff, though the animals remained behind.
Local officials and residents said it was the worst fire in the Hollywood Hills in 50 years and feared it was a sign of things to come as Southern California suffers through its driest spring on record.
''We are facing an incredibly difficult fire season,'' Los Angeles Mayor Antonio Villaraigosa said yesterday.
Gov Arnold Schwarzenegger has issued an executive order to mobilize more personnel and equipment to respond to wildfires.