Five fire engines attended the blaze which was confined to a single building.
No people were injured in the fire which began around 8pm on Tuesday.
The cause of the blaze has not been determined.
A spokesperson for Lancashire Fire and Rescue Service, said: “An estimated 21,000 chicks have perished in a fire in a single-storey building measuring 70 metres by 20 metres floor plan.
“Five fire engines and an aerial ladder platform (ALP) and crews were assigned, two from Preston, one from Fulwood, one from Penwortham and ALP and support pump from Blackpool.
“Firefighting is still under way, with the use of two main jets and a hosereel. No people have been injured. The cause of the fire is to be established.”