The incident was reported at 12:47 p.m. on southbound Highway 29 just north of Tubbs Lane in an unincorporated area.
A bus somehow caught fire and sparked a grass fire along the roadway, CHP Officer Kevin Kroncke said.
He said there were reports of explosions coming from the bus, which was fully engulfed in flames.
It appears the occupants of the bus were able to exit, Kroncke said. He did not have word on injuries.
He said Pacific Gas & Electric Co. crews have been sent to the scene to deal with any problems related to power lines.
The roadway was shut down around 1:20 p.m. and the closure was expected to last about an hour.