The serology of 138 strains of Vibrio fluvialis was studied. Eighteen O-antigenic groups were defined among them and it was shown that the H antigens of all the strains were identical regardless of the biovar. The presence of mucoid antigen, which inhibits O agglutination, was found in some strains. As all O antisera for V. fluvialis contained some R antibody, all diagnostic O sera must be absorbed with R organisms before use. Some O antigens of V. fluvialis were identical with those of certain Vibrio cholerae serovars.