Aquaculture and fishing in India traditionally have strong export markets. Various kinds of marine and fresh water fish have been cultured and the worldwide production of cultured fish increases every year .But fishes have intimate contacts with their environment which can contain very concentration of bacteria, viruses and fungi. One of the major pathogen in India, Aeromonas hydrophila, is known to cause a variety of diseases in fish. Intensive aquaculture practices have led to a growing interest in understanding fish diseases, so that they can be prevented or treated. India is a rich source of traditional herbs with more than 500 listed plant species many endowed with broad spectrum antimicrobial properties. Plant extracts can act as immunostimulant even at low concentrations and hence, its use could be very cost effective. The water extracts of A. marmelos for their immunostimulatory effect on the antibody response to heat killed A.hydrophila in O.mossambicus was tested using Bacterial agglutination assay. Generally the lower doses of leaf extracts administered group significantly stimulated the antibody response when compared with control.