This paper reports two different enterotoxigenic classes of the genus Aeromonas isolated from fishes. Strains of A. sobria were isolated from healthy fishes only whereas strains of A. hydrophila were isolated from both healthy and moribund fishes. Aeromonas sobria and hydrophila strains produced a cytotoxic factor and were highly proteolytic. Seventy-five percent (75%) of the A. sobria strains produced only one type of hemolysis on 5% blood agar, but 92% of the A. hydrophila produced two types of hemolysis. Enterotoxigenicity of all strains was tested by the rabbit gut loop technique and the suckling mouse test. Most of these Aeromonads were enterotoxigenic. Enterotoxigenic strains appeared to produce two different enterotoxins which were antigenically related, but they seemed to have different mechanisms of action. Enterotoxins of Aeromonas were shown to be antigenically different from those of E. coli.