In October 1992, a new strain of cholera, subsequently designated Vibrio cholerae O139 Bengal, was detected in Madras, India. This strain spread rapidly through the Indian subcontinent and has now been reported in many parts of Asia, with additional cases identified in travelers to North American and the Middle East. Phylogenetically, V. cholerae O139 Bengal is very closely related to "standard" V. cholerae O1 El Tor strains; it produces cholera toxin and causes an illness identical that seen with V. cholerae O1. However, prior immunity to V. cholerae O1 El Tor does not appear to protect against illness caused by V. cholerae O139 Bengal. O139 Bengal strains have a short, "semi-rough" O side chain and are encapsulated, changes that are likely to have accounted for their ability to cause disease in persons with prior exposure to cholera. These changes in surface structures appear to have resulted from a limited number of genetic modifications. The appearance of V. cholerae O139 Bengal may well herald the beginning of the eighth pandemic of cholera--and underscores the tremendous potential within nature for creation of new strains of "old" pathogens.