Two Bacillus species isolates (B₁ and B₂),showed in-vitro antagonistic activity against Aeromonas hydrophila isolated from the intestinal tract of apparently healthy Oreochromis niloticus. Both antagonistic isolates were identified as Bacillus licheniformis and Bacillus cereus/thuringiensis, respectively by using Biolog's microbial identification system, and they were evaluated to be safe to O. niloticus when inoculated intra-peritoneal (I/P). Feeding experiments were carried out in-vivo to investigate the effect of both antagonistic isolates (1 X 10⁷ CFU/g diet) either alone or combined on fish immune response and resistance to A. hydrophila infection. Immunological parameters (phagocytic, lysozyme and serum bactericidal activities) were evaluated as well as a challenge test using pathogenic A. hydrophila. Results revealed that, antagonistic Bacillusisolates either alone or combined triggered significant (P ≤ 0.05) increase in phagocytic, lysozyme and serum bactericidal activities; with the highest values in fish received a mixture of both antagonistic isolates. Survival of A. hydrophila challenged fish was highest in fish fed on both antagonistic isolates, followed by B. cereus/thuringiensis, and then B. licheniformis fed fish.