A flagellar sheath protein of Vibrio cholerae CA401 (Inaba) was characterized. Purity of the preparation was indicated by a single band on polyacrylamide gel electrophoresis gels and on Ouchterlony plates prepared with antibody against crude sheath material. The sheath protein was composed of three polypeptides with minimal molecular weights of 61,500, 60,000, and 56,500. The presence of sheath protein on the flagellum as well as on the outer membrane of the cell was demonstrated by ferritin labeling experiments with antiserum. Sheath protein antibody reacted similarly in labeling experiments and agglutination tests with a classical Ogawa strain and two nonagglutinating V. cholerae isolates, indicating that the sheath protein may represent the common Vibrio H antigen. Antibody specific for lipopolysaccharide labeled the cell but not the sheathed flagellum, which demonstrated that the sheath is not a simple extension of the outer membrane of the cell.