We conducted studies to investigate the surface architecture of V. cholerae O139 using electron microscopy and compared it with O1 and other serogroups of V. cholerae. The bacterium is comma-shaped and has a single polar flagellum and morphologically resembles the classical and E1Tor biotypes of V. cholerae O1. High power electron microscopy showed a few pili, 5 to 7 nm in diameter, and 2 to 3 in number per bacterium. The presence of a capsule on electron microscopy of ultrathin sections of V. cholerae O139 treated with polycationic ferritin clearly distinguished the O139 serogroup from the O1 serogroup which are not encapsulated. Immunoelectron microscopy further revealed that an anti-O139 monoclonal antibody of the IgG2a isotype bound specifically only to an O139 strain but not to any other serogroup of V. cholerae indicating that O139 has unique epitopes not found in other serogroups of V. cholerae.