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"Actinomycetes were isolated from samples of seawater, sediment, seaweeds and swabs taken from submerged substrates collected from five sampling stations viz., Mangrove Swamp, Roche Park, Thermal Beach, Hare island and Inshore area along the coast of Tuticorin. About 133 actinomycetes strains were isolated from 129 marine samples. Of the 104 actinomycete isolated screened for their inhibitory activity against the test organisms viz. Edwardsiella tarda, Pasteurella sp., Vibrio vulnificus, Salmonella typhi, S. typhimurium and Escherichia coli, 77 (74%) isolates were found to be inhibitory. Antagonistic actinomycetes were found to be more prevalent in sediment samples. Among the sampling stations, higher numbers of inhibitory strains were isolated from Thermal Beach. All the isolates of antagonistic marine actinomycetes were identified to belong to the genus Streptomyces. The antagonistic marine Streptomyces strains expressed wide antibacterial spectrum against fish bacterial pathogens viz. Aeromonas hydrophila, A. sobria, E. tarda, V. fischeri, V. vulnificus and Pasteurella sp. The ethyl acetate crude extract of the antibacterial substances obtained after extraction with ethyl acetate from the inhibitory Streptomycetes strains displayed inhibitory activity against the fish pathogens such as A. hydrophila, E. tarda, and Pasteurell sp. The results of the present study clearly indicate that antagonistic marine Streptomycetes strains could be used to prevent and/or control fish diseases caused by bacterial pathogens in fish culture systems."