The bactericidal and virucidal effects of hypochlorite produced by electrolysis of salt water were examined for pathogenic bacteria and viruses of fish. A three percent solution of sodium chloride that contained pathogenic bacteria or virus was electrolyzed and the organisms were exposed to hypochlorite. More than 99.99% of the cells of Vibrio anguillarum and Aeromonas salmonicida, the causative agents of fish vibriosis and furunculosis respectively, were killed when the bacteria were exposed to 0.1 mg/l hypochlorite for one minute. Yellowtail ascites virus (YAV) and hirame rhabdovirus (HIRRV), the causative agents of viral ascites disease and rhabdovirus disease of marine fish, were inactivated >99.99% after treatment with 0.58 mg/l hypochorite for one minute. The bactericidal and virucidal effects of hypochlorite produced by electrolysis (3.5 m3/h, 0.1 A) are greater than those of the chemical reagent. The purity of the sodium chloride used influences the production of hypochlorite. Sodium chloride obtained as a super grade chemical reagent is more effective than sodium chloride that is used for food. However, a sufficient disinfectant effect was observed in electrolyzed seawater, which could be applied in aquaculture.