Cal Fire officials said a vegetation fire southwest of Tracy is 50% contained Wednesday night.
As of about 8 p.m., the Hollow Fire had burned 283 acres as crews including those from the Alameda County Fire Department sought to extinguish it.
The acreage was revised downward from 500 acres due to better mapping, Cal Fire spokeswoman Pam Temmermand said.
"The plan right now is to strengthen the containment lines," Temmermand said.
She said firefighters will be out there Wednesday night until they get that done.
The fire is burning grass and brush off of West Corral Hollow Road in the Carnegie State Vehicular Recreational Area in San Joaquin County. Cal Fire said on social media that the terrain is causing firefighters some difficulty.
Temmermand said no one has been injured and no structures have been damaged. The fire is burning in "cattle country with the occasional structure here and there," she said.
The fire was first reported shortly after 3 p.m. The cause is under investigation.