In the past few decades, the search for alternative strategies for disease management other than chemotherapeutants has been encouraged. Recently, disease prevention among fishes depends mainly on the use of vaccine and immunostimulants together with optimal husbandry conditions. The present study examined the effects of Nigella sativa (NS) on the response of catfish (Clarias gariepinus) to vaccination and/or challenge infection. For this purpose, four hundred catfish allocated into four groups with two feeding regimes (basal diet (G1 & G2) and diet supplemented with NS at a dose level of 3 g/100 g diet (G3 & G4). The groups (G2 & G4) are intraperitoneally (IP) vaccinated with 0.1 ml formalin-killed Aeromonas hydrophila bacterin. The effects of NS in G3 and G4 are evaluated in comparison with other groups (G1 & G2) using hematological and biochemical parameters together with histopathological examinations. Six weeks after the start of the experiment, the results revealed a significant increase in erythrogram (RBCs, Hb and PCV%), Leucogram (total Differential leucocytic count), serum total protein and globulin in catfish supplemented with 3% NS. On the other hand, no significant changes in liver enzymes, urea, uric acid and creatinine among all experimental groups were noticed. The histopathology revealed proliferation of hematopoietic tissues and the activation of melanomacrophages in both supplemented and vaccinated groups. Two weeks after the challenge experiment, vaccinated groups (G2 and G4) showed a significant increase in the antibody titer while treated groups supplemented with 3% NS showed a significant increase in the phagocytic activity. After the challenge, catfish vaccinated and supplemented with 3% NS showed the highest (95%)relative level of protection (RLP) followed by a vaccinated group (85%) then supplemented group(55%). Nigella sativa (3 g/100 g diet) enhances the immune response and resistance of catfish to A. hydrophila and consequently has a potential value for aquaculture.