Apparently healthy Oreochromis niloticus (n = 225) were used to investigate the effect of commercial probiotic (B. subtilis) or fish gut isolated Bacillus strains (B. amyloliquefaciens; 7 H N and B. cereus; 29 H N) on growth performance, immune response, some serum biochemical parameters and disease resistance against Aeromonas hydrophila infections. They were randomly assigned to four treatments with three replicates per treatment (45 fish/ treatment; 15 fish/replicate except control group has six replicates). The experimental fish were fed with diets supplemented with 0 (control), 1 x 10⁶ CFU g^-1 fed commercial B. subtilis or gut isolated B. amyloliquefaciens (7 H N) and B. cereus (29 H N)  for 2 months then bacterial challenge were performed at the end of the experiment. Fish fed gut isolated B. cereus (29 H N) had the highest marked (p<0.05) influence on the productive performance parameters among treated groups. Furthermore, the highest levels of nitric oxide, Immunoglobulin (IgM)  and lysozyme were found in treated groups. Infected fish fed on gut isolated B. cereus had a positive effect on ALT, SOD activity while creatinine and urea were marked reduction compared with other dietary treatments. It could be inferred that gut isolated Bacillus has been suggested as growth promoter, immune-stimulant in O. niloticus and increase protection against A. hydrophila infection. Among the two isolated Bacillus species, dietary supplementation with the B. cereus had the highest performance in O. niloticus compared with the commercial B. subtilis.