AbstractImmunostimulants increase the immunocompetency and disease resistance of fish by enhancing immune responses. This study determined the in vitro and dietary effects of chitin on immune responses of catla, Catla catla. Catla head‐kidney leukocytes were cultured in complete RPMI‐1640 medium containing 0.01, 0.1, and 1.0 mg/mL of chitin. Leucocyte viability and different cellular immune responses, namely, respiratory burst, nitric oxide production, and leucocyte peroxidase content were analyzed in vitro. In the dietary experiment, chitin‐supplemented diets (0.5, 1.0, and 2.0% of feed) were fed to catla for 2 wk, and respiratory burst, serum lysozyme, and total protein contents were measured after 1 and 2 wk of feeding. Leucocytes incubated in vitro with chitin for 72 h maintained their viability. Chitin significantly (P < 0.05) enhanced all the immune responses in vitro. The lowest dose showed the best response. Dietary chitin also significantly stimulated all the immune parameters in a dose‐dependent manner with the highest dose showing the maximum responses. All the responses were higher after 1 wk of feeding and declined thereafter. The results suggest that chitin stimulates catla cellular and humoral immune responses in vitro and in vivo.