Plesiomonas shigelloides is often regarded as a non-pathogenic bacterial species that is occasionally isolated from patients with diarrhoea. However, a review of travellers returning to Japan with diarrhoeal illness through Kansai Airport revealed that the incidence of P. shigelloides from microbiologically confirmed cases increased from 23·2% in 1987 to 77·8% in 1999. We carried out a descriptive epidemiological study to identify patterns associated with diarrhoea due to this organism. Selected P. shigelloides isolates from this patient group were compared by pulsed-field gel electrophoresis of SpeI total chromosomal DNA digests to determine their genetic heterogeneity. Over the study period (whole of 1996 and first 2 months of 1999), 1149 of 1659 (69·3%) patients with microbiologically confirmed gastroenteritis yielded P. shigelloides. Infection was characterized by watery diarrhoea five times per day that persisted for 3 days. No statistically significant association was found between factors such as age, gender, destination, length of trip, but multivariable logistic regression analysis revealed an association between additional symptoms (vomiting, fever, abdominal pain) age and gender. The molecular fingerprints of a selection of 39 isolates and 3 reference strains of P. shigelloides were highly variable and each had a unique profile. We conclude that although P. shigelloides infections are usually mild and self-limiting, this organism may contribute to a significant proportion of travellers' diarrhoea in the Orient. The species is characterized by great heterogeneity at the DNA level.