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Solid state fermented pigeon pea meal (FPP) was used in substituting fish meal (FM) in diets of Oreochromis niloticus. Five diets varying in inclusion levels of FM: FPP as followed: Feed 1, 250:50; Feed 2, 200:100; Feed 3, 150:150; Feed 4, 100:200 and Feed 5, 50:250 were made. A commercial feed (Feed 6), was used as a control. Triplicate groups of tilapia fingerling were stocked in plastic aquaria at six fish per unit and fed ad libitum for 60days. Results showed that specific growth rate (SGR) of tilapia fed Feed 5 (2.60±0.12% day-1) was better (p<0.05) than those recorded in Feed 1 (2.36±.07 % day-1). Similarly, food conversion ratio (FCR) (1.13±0.01) and mean weight gain (60.00±0.04g) were higher (p<0.05) in Feed 5 when compared with those in Feed 1 where FCR was 1.19±0.02 and MWG (50.96±0.04g). The results of Aspartate amino transferase (AST) and alanine amino transferase (ALT) decreased with increasing inclusion level of FPP. Gut microbial load increased with increasing level of FPP. The fish group fed Feed 5 had the highest foregut (2.60 x 10-5 CFU/ml), midgut (3.98 x 10-5 CFU/ml) and hindgut (4.52 x 10-5 CFU/ml) micrbiota. In general, gut microbiomes were dominated by cellulose and carbohydrate utilizing bacteria: Citrobacter fruendi, Bacillus subtilis and Staphylococcus aureus. Inference from the study revealed that O. niloticus was able to utilize the diets due to the presence of probiotics in them.