An unusually high incidence of Vibrio cholerae O1 infection was observed in southern Thailand between late December 1997 and March 1998. Fifty-seven V. cholerae O1 strains were isolated in five provinces during this epidemic and were examined. They were El Tor Ogawa strains exhibiting similar antibiograms. All strains were resistant to tetracycline, which had not been reported in Thailand since 1993. The ribotypes, hybridization patterns with ctx and zot gene probes, arbitrarily primed PCR profiles, and pulsed-field gel electrophoresis profiles of the representative strains were compared with the clinical strains isolated from patients in India and Bangladesh in 1997 and 1998 and from international travellers originating from various Asian countries during the 1992–8 period. All southern Thailand strains and the 1998 international traveller strain of Thai origin showed indistinguishable genetic fingerprinting patterns that were distinct from those of other test strains. The results suggest that a tetracycline-resistant clone newly emerged in late December 1997 caused the large epidemic in southern Thailand and that the variants with a slightly different antibiogram appeared during the course of the spreading epidemic.