An 0 agglutinable mutant was isolated from the ultraviolet-irradiated culture of Vibrio parahemolyticus. A-55 (Type 13). This mutant retained the flagellar H and somatic O antigen of its parent strain but lacked in a specific antigenic component which was considered as the K (Kapsel) antigen. The behavior against heat and HC1 of the K antigen, as revealed by agglutination and absorption tests and by antibody formation in the rabbit, resembled that of the B antigen found in certain ofE. coli. The O antigen of the mutant, naked of the K antigen, was destroyed by a treatment with HC1 resembling 5 antigen in certain of theSalmonella.