The blaze was reported shortly after 5 p.m. in an open field on the westbound side of state Highway 12 just west of Summerset Drive near Rio Vista, California Highway Patrol Officer Tom Stewart said.
It was not immediately known what type of structures are on fire or how far the fire has spread in the field, according to the Solano County Sheriff's Office.
No injuries have been reported, the sheriff's office said.
The CHP issued at Sig-alert at 5:58 p.m. for Highway 12.
