Of 200 isolates of Vibrio mimicus screened, one from water (N-57) agglutinated with V. cholerae O139 polyclonal antiserum (absorbed with a rough strain of V. cholerae only) and not with O139 polyclonal diagnostic antiserum (absorbed with the rough strain and V. cholerae O22 and O155). The antigenic relationship between V. cholerae 0139 and N-57 is of a,b-a,c type, where a is the common antigenic epitope and b and c are unique epitopes. Strain N-57 was assigned to a new serogroup of V. cholerae O194. It gave negative results in a monoclonal antibody-based rapid test and a PCR test specific for V. cholerae O139. It did not possess the ctx gene or produce cholera toxin. Antiserum to strain N-57 cross-protected infant mice against cholera on challenge with V. cholerae O139. Structural studies of the surface polysaccharides and studies of the rfb genes will shed more light on the extent of relatedness between V. mimicus N-57 and V. cholerae O139.