The fire broke out at around 8pm in a building on Rosemary Lane, Salwick.
A spokesman for Lancashire Fire and Rescue Service said no people were injured in the fire, which is still ongoing as of 9.45pm this evening.
A spokesman 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."