In a survey examining the causes of travellers' diarrhoea treated in Tokyo between July 1986 and December 1995, Aeromonas species were isolated from 1265 (5·5%) of 23215 travellers returning from developing countries. Aeromonas species were the fourth most frequent enteropathogen isolated, following enterotoxigenic E. coli (8·5%), Salmonella spp. (7·6%) and Plesiomonas shigelloides (5·6%). Aeromonas species were found in 1191 (5·6%) of 21257 patients with diarrhoea and in 74 (3·8%) of 1958 healthy individuals without diarrhoea. Mixed infection was observed in 512 (40·5%) cases. No significant difference in the prevalence of Aeromonas by year, season, age distributions, or sex was observed, but a slight difference was noted depending on the country where the travellers visited. Of the 1265 Aeromonas isolates, 893 strains (70·6%) were A. veronii biovar sobria, 330 (26·1%) were A. hydrophila, and 42 (3·3%) were A. caviae. The clinical symptoms of patients from whom Aeromonas species was isolated as the only potential enteric pathogen were almost similar, which were watery diarrhoea (about 60%), abdominal cramps (43%), fever (around 15%), and nausea or vomiting (13%). Although the severity of illness was milder than that of enterotoxigenic E. coli alone, these data suggest that Aeromonas species are important enteric pathogens in travellers' diarrhoea.