There is need to reduce usage of antibiotics in favor of prebiotics and phytogenic in aquafeed production. The study evaluated effects of mushroom on growth, gut histology and haematological variables of Clarias gariepinus. Five treatment feeds were made to vary in inclusion of solvent extracted oyster mushrooms, Lentinus sajor-caju as follows: Feed 1, 0%; Feed 2, 4%; Feed 3, 8%; Feed 4, 12% and Feed 5, 16%. The diets were made isonitrogenous (42%) and isolipidic (6.0%). The growth performances, gut histology and haematological profile of the fish were estimated using standard procedures. The essential amino acid (EAA) contents of the mushroom and feeds were analyzed following standard methods. Ingredients were mixed according to proportions, preconditioned and steam pelleted. African catfish of average weight 9.48g were stocked in three replicates 20liter aquarium, at 113.76g/aquarium and fed with the experimental diets for 90days. Results showed that mushroom had good array of EAA especially lysine (7%). The haematobiochemical parameters and growth of the catfish were best at 8% mushroom extract inclusion. Histological sections showed significant increase in length and width of the mucosal fold microvilli as mushroom extract inclusion increased up to 8%. Similarly, red blood cell counts, packed cell volume, hemoglobin and total white blood cell counts of the catfish increased up to 8%. Fish fed Feed 3 had the best specific growth rate (3.00±0.06% day-1), food conversion ratio (1.54±0.01), weight gain, (128.67±0.02g) and protein conversion ratio of 0.3±0.04. Catfish fed with higher than 8% inclusion of mushroom had reduced growth rates, hemato-biochemical and histological parameters. This may be probably due to the high polysaccharides and beta-glucans present in mushroom. Thus, mushroom extracts should be included in catfish feed but not beyond 8%. [ABSTRACT FROM AUTHOR]