We collected diarrhoea specimens in two hospitals in southern Thailand in 1999 to examine whether infection by the Vibrio parahaemolyticus pandemic clone is prevalent. V. parahaemolyticus was isolated from 317 specimens. Seventy-six per cent of the isolated strains had the pandemic clone-specific characteristics (tdh+, trh-, and an unique toxRS sequence detectable by GS-PCR) and an associated characteristic (the ORF8 sequence of f237 phage). These strains belonged to the three pandemic servovars with the O3:K6 strains being dominant and three other serovars (O1:K25, O1:K41 and O4:K12). We also found O1:K25 and O1:K41 strains with the pandemic clone-specific characteristics among the strains isolated from the international travellers who left Thailand and three other Asian countries between 1998 and 1999, verifying pandemic potential of these strains. The results demonstrate prevalence of infection by the pandemic clone in southern Thailand and suggest emergence of various serovariants in this area and their implication in international spread.