State fire spokeswoman Stacie McCambridge says the blaze began shortly before noon Tuesday in the Pope Valley area of Napa County. It grew to 2,700 acres by late Tuesday.
State firefighters and crews from Napa, Lake and Solano counties spent the day working in 90-degree weather as they contained 30 percent of the fire.
McCambridge says several residents living in two communities have been evacuated because nearly 200 homes are threatened.
Five structures have also been damaged, but it wasn't immediately clear whether they were homes or other buildings.
No injuries have been reported. The cause of the fire is unknown.