Three isolates of pathogenic Treponema hyodysenteriae, B8044, B6933, and Ack 300/8, were serotyped by double immunodiffusion using lipopolysaccharide antigen and rabbit antiserum. The precipitation reactions of these three isolates were then compared with precipitin reactions from the reference strains of serotypes 1 through 4. The data indicated that B8044, B6933, and Ack 300/8 share antigenic determinants with either serotype 1 or 2 or both. However, when the cross-reacting antibodies were removed by adsorption, all reactions were found to be monospecific. Based on these results, three new serotypes of pathogenic T. hyodysenteriae are proposed: B8044 serotype 5, B6933 serotype 6, and Ack 300/8 serotype 7.