The fire was reported in the area of Torrington Way and Mystic Drive shortly before 6 p.m., Fairfield Fire Chief Vincent Webster said.
The flames have climbed to the crest of a hill and consumed around 88 to 90 acres so far, Webster said.
The area is relatively remote and no structures are threatened, but firefighters are trying to protect a communications tower at the top of the hill.
Webster said the edges of the fire are largely contained and conditions are good, with minimal wind, but firefighters are working to keep the fire from continuing down the other side of the hill.
Around 80 to 100 firefighters are on the scene and Cal Fire is providing aerial support.
