Crews battling the Diablo Fire near Byron have the blaze 50 percent contained on Sunday afternoon, Cal Fire said.
The brush fire was reported about 9:20 a.m. near Vasco Road and Camino Diablo in Contra Costa County.
The blaze had burned 128 acres and was 50 percent contained, Cal Fire said on social media about 1:45 p.m.
No injuries have been reported and no structures have been damaged. The cause is under investigation.