A total of 182 isolates of motile Aeromonas from patients with diarrhea and environmental sources was investigated for hemolytic activity to rabbit erythrocyte and cytotoxicity to HeLa 229 cell. Furthermore, the relation between O serogroup and virulence factors, which were lethal to mouse and autoagglutination, were investigated. There were many strains possessing both the hemolytic and cytotoxic activities in A. hydrophila from overseas traveller's diarrhea, suggesting that these activities were associated with intestinal pathogenecity. Although there was a clear correlation between hemolytic and cytotoxic activities in A. hydrophila from overseas traveller's diarrhea, the correlation was not found in A. hydrophila from domestic cases of diarrhea and A. sobria from overseas traveller's diarrhea. Especially, some A. hydrophila isolates from domestic case of diarrhea produced only hemolysin. These results indicated that there was a difference in specificity between the toxins accounted for hemolytic and cytotoxic activities, and more than two different toxins were developed. O serogroups 11, 34, 14, 16, and 35 in that order were the most frequent serogroups. About half of O11 and O34 strains possessed lethal activity to mouse. Autoagglutination phenomenon did not seem to be associated with the lethal activity. In O11 strains, high cytotoxic titer was more frequently found in lethal activity positive-strains than in the activity negative-strains, suggesting that cytotoxicity contributed preferentially to lethal activity to mouse. But such a correlation was not found in O34 strains, so other virulence factors than hemolysin and cytotoxin may be associated with the lethal activity.