Seventy clinical strains of Vibrio cholerae O1 isolated from 1982-96 in Samutsakorn, a port city 30 km southwest of Bangkok where cholera occurs at low levels with regular seasonality, were characterized to investigate if there were any differences among the O1 strains isolated before, during and after the 0139 epidemic. Pulsed-field gel electrophoresis (PFGE) typing, ribotyping and southern blot hybridization with a cholera toxin probe (CT genotyping) demonstrated several genotypes among O1 strains isolated before the emergence of V. cholerae 0139. However, O1 strains isolated during and after the advent of 0139 showed identical ribotypes which were distinctly different from the types identified in strains isolated before the emergence of 0139. Ribotypes identified in strains during and after the advent of 0139 were also demonstrated by O1 strains isolated immediately before the emergence of 0139. Considering the seasonality of cholera in Samutsakorn, the identical ribotype and CT genotype and the closely related PFGE types shown by all O1 strains isolated during and after the appearance of 0139 is remarkable and suggest that the V. cholerae O1 strain may reemerge from an environmental source. A subgroup of V. cholerae O1 strains isolated before the emergence of the 0139 epidemic had a ribotype identical to a type demonstrated by 0139 strains isolated in Thailand. Our results support similar findings in Bangladesh and India that a distinct O1 strain appeared during the 0139 epidemic. However, compared with the apparent identical strain which replaced 0139 in Bangladesh and India, the emerged O1 strain in Samutsakorn showed a different ribotype and CT genotype.