Fingerlings of Clarias gariepinus were used to evaluate the effect of dietary fumonisin B1 (FB1), a mycotoxin produced by Fusarium verticillioides, on growth, haematological and serum biochemical parameters. The fingerlings were sorted, weighed and randomly stocked in 16 plastic tanks at the rate of 20 fingerlings per tank. Fusarium-cultured maize grains containing FB1 were used to formulate three diets containing approximately 5.0, 10.0 and 15.0 mg FB1/kg, constituting diets 2, 3, and 4 respectively. These three diets, plus diet 1, which contained non-Fusarium cultured maize grains that served as the control, were used in a 6-week feeding trial. The final weight gains by the fingerlings were significantly (P