Vibrio parahaemolyticus is a major cause of gastroenteritis worldwide. A total of 95 V. parahaemolyticus isolates belonging to 23 different serovars were identified in a case-control study of expatriates and Thai adults from 2001 to 2002 in Thailand. Fifty-two per cent of isolates (49/95) were resistant to ampicillin and sulfisoxazole, but all isolates were susceptible to ciprofloxacin and trimethoprim-sulfamethoxazole, two antibiotics commonly used to treat traveller's diarrhoea. All isolates were positive for the species-specific toxR gene, and 91 and 5 were positive for the thermostable direct haemolysin (tdh) gene and the tdh-related (trh) gene, respectively. Sixty-five isolates were assigned to the pandemic group of V. parahaemolyticus by a group-specific PCR and the presence of the orf8 gene. The pandemic isolates belonged to three recognized serovars (O3 : K6, O1 : K25, O1 : KUT) and a new serovar, O3 : K46. This new serovar harboured pandemic traits. PFGE analysis revealed that all pandemic isolates including serovar O3 : K46 were closely related and clearly distinct from the non-pandemic isolates. In summary, three well-known serovars of pandemic V. parahaemolyticus isolates were identified as a major cause of diarrhoea in Thailand and a new V. parahaemolyticus isolate, serovar O3 : K46, with pandemic traits was detected.