AbstractOutbreaks of fish diseases have been reported in many parts of the world. The outbreaks are difficult to control. During December 1982 and March 1983, there were severe outbreaks of fish disease in Thailand. A study on the presence and inactivation of harmful toxins, i.e., cytotoxin and hemolysin, by heat, salt and gastric pH was undertaken. Cytotoxin and hemolysin were detected in all diseased snakehead fish (Ophicephalus striatus) homogenates.Aeromonas hydrophilaF 588 isolated from the diseased snakehead fish also produced cytotoxin and hemolysin. No detectable cytotoxic or hemolytic activity was found in the fish homogenates orA. hydrophilaF 588 cell suspensions after heating at 100°C for 5 minutes or autoclaving at 121°C for 15 minutes at 15 lb/in.2Cytotoxic activity remained positive in all concentrations (0% to 30% W/V) of NaCl. However, no cytotoxin could be detected when the pH of the samples was 2.0. There was a 55% decrease in hemolytic activity whenA. hydrophilaF 588 was incubated in 30% NaCl for 1 month at 30°C. Furthermore, there was a 50% decrease in the activity when the pH of the samples was 2.0. Hence, the diseased fish is safe for consumption if it is heated for 5 minutes at 100°C. However, it is unsafe to consume unheated salt‐fermented