The incident happened at the location near Teilman and Delno just before 10 am Friday.
Fresno Fire officials say they are working to control a high-pressure CO2 leak. The leak is in the back of the plant.
Officials say there is no immediate threat to the public.
No injuries have been reported, but ambulances are at the scene as a precaution.
A risk for the firefighters is the extreme cold temperatures from the gas, which crews are working to shut off.
A spokesperson with Foster Farms responded to the incident, writing:
"Early this afternoon, there was a Co2 leak at the Foster Farms Belgravia plant in Fresno. The company notified the fire department who assisted with the assessment and control of the Co2 leak. The plant was safely evacuated, and all employees were sent home. There have been no injuries to Foster Farms employees or members of the surrounding community. We expect the plant to resume operations on Saturday, December 24."