The fire, which is burning in remote grassy oak woodlands that are difficult to access, is threatening 180 structures. It is centered near the intersection of Snell Valley Road and Butts Valley Road about eight miles southeast of Middletown in unincorporated Napa County.
WATCH: Video of the fire captured by an ABC7 News viewer
Fire prompts mandatory evacuations near Middletown in Napa County
The fire, which is being fought by 480 firefighters, was first reported around 2:30 p.m. Saturday. No structures have been damaged so far, and no injuries have been reported.
RELATED: Napa County fire threatens 180 homes
Get the latest stories and videos on wildfires burning across California, here.