Abstract: The loss of fish production from infectious diseases accounts about 60% of all diseased fish. One of the major constrains for the development of aquaculture is the loss due to disease. So, we need to modulate the immune system of fish. A study has been carried out to investigate the development of immunity by extract of medicinal plant Ocimum sanctum on different concentrations (10, 20 and 30 ppm) against heat killed A. hydrophila on the fingerlings of common carp, Cyprinus carpio. In the plant extract treated fishes, no mortality was seen. Increase in the weight of the fish observed in plant extract treated groups, (10 ppm) showed more growth than the control and other groups. The low dose (10 ppm) of plant extract showed maximum antibody response and more phagocytic activity than the control and other experimental groups. The 10 ppm was found more efficient. Hepatosomatic index was more in the control group due to the enlargement of liver. However, the plant extract administered group showed decreased HSI.