A total of one thousand Oreochromis niloticus held in earthen ponds of the WorldFish Center were used in this study. Forty of these were randomly collected from the ponds and subjected to bacteriological and pathological examination where a variety of 80 bacteria were isolated. Among those 15 bacterial isolates were examined for their probiotic activity through studies performed in vitro and in vivo. Aeromonas hydrophila, Citrobacter freundii, Pseudomonas species and two Gram-positive isolates (Bacillus pumilus and Bacillus firmus) were identified. A total of 120 O. niloticus were used to evaluate the pathogenicity of Aeromonas hydrophila and 240 O. niloticus were used to assess the safety of the isolated probiotics. Three out of the 15 isolates examined (B. pumilus, B. firmus and C. freundii) showed inhibitory effects against A. hydrophila in vitro and did not cause either disease sings or mortalities in case of B. pumilus and B. firmus when injected into the fish. Diets were prepared and supplemented with isolates which showed the greatest promise as probiotics. The viability of the bacteria in the diets was assessed following storage of the diet at 4 and 25 °C for five weeks. B. pumilus survived at either 4 °C or 25 °C for five weeks. B. firmus and C. freundii survived at 4 °C at week 5 and week 4 of storage respectively, but they were no longer viable in the feed stored at 25 °C after 2 and 1 weeks of storage, respectively. Feeding experiments were conducted on 600 O. niloticus using the diets containing single ormixed isolated probiotic bacteria. A challenge test was carried out on the fish fed supplement diets using pathogenic A. hydrophila to assess their disease resistance to the bacterium. Fish that received feed supplemented with probiotics showed no evidence of disease after challenge infection. Survival was highest with fish fed diets supplemented with B. pumilus, followed by a mixture of probiotics, and then C. freundii. B. pumilus appears promising as a probiotic for controlling A. hydrophila infection among O. niloticus.