A large cluster of virulence genes encoding proteins involved in Vibrio cholerae accessory colonization factor (ACF) expression and toxin-coregulated pilus (TCP) biogenesis is flanked by sequences that resemble bacteriophage attachment (att) half-sites. Adjacent to the attL-like site is a gene (int) that encodes a protein related to the integrase family of site-specific recombinases. The putative vibrio integrase appears to be most closely related to the Escherichia coli cryptic prophage (CP4-57) integrase protein (52% identity, 73% similarity). Genomic analysis of numerous V. cholerae strains (O1, non-O1 and O139) revealed that only vibrios capable of causing epidemic Asiatic cholera possess the TCP-ACF colonization gene cluster in association with the integrase. The fact that the integrase gene is absent in avirulent strains suggests that epidemic strains of V. cholerae obtained the TCP-ACF colonization gene cluster via horizontal transfer.