To further understand the epidemiology of enterotoxigenic Bacteroides fragilis (ETBF), 89 extraintestinal B. fragilis strains from Seoul, Korea, were examined for secretion of B. fragilis toxin (BFT) by the HT29/C1 biologic assay and for the B. fragilis toxin gene (bft) by colony blot hybridization and PCR. Complete agreement between the three techniques was found. Overall, 34 B. fragilis strains (38%) were identified as ETBF. Eleven of the 34 ETBF strains (32%) expressed a new isoform of BFT (Korea-BFT). This new isoform is more related to BFT-2 than to BFT-1. Like BFT-1 and BFT-2, Korea-BFT cleaves E-cadherin, the zonula adherens protein.