Abstract: As a major pathogen for fish, the antimicrobial activity of Diallyl Disulfide (DADS) was examined for the following bacteria, Aeromonas hydrophila, A. salmonicida ssp. masoucida, A. salmonicida ssp. salmonicida, Edwardsiella tarda, Vibrio vulnificus, V. paraheamolyticus and L. anguillarum. About 10 ug mL-1 and more of DADS formed a clear inhibitory zone to all pathogenic bacteria in a disk diffusion test. The Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) values were in the ranges of 160-640 and 640-1280 ug mL-1 of DDS, respectively. The most sensitive pathogen to DDA was V. vulnificus (160 ug mL-1 for MIC and 640 ug mL-1 for MBC) followed by E. tarda (320 ug mL-1 for MIC and 640 ug mL-1 for MBC). These results suggest bioavailability of DADS for controlling bacterial pathogens in aquaculture.