Erysipelothrix rhusiopathiae was isolated from tonsils of 63 (10.5%) of 600 apparently healthy slaughter pigs in the Kanto area of Japan in February and July 1984. The isolation rate was significantly higher during July than in February. Of these 63 isolates, 34 isolates (54.0%) were serotype 7, 20 isolates (31.7%) were serotype 2, 6 isolates (9.5%) were serotype 6, and 1 isolate (1.6%) each was serotype 11, 12, or 16. All isolates of serotypes 2, 6, 11, 12, and 16 were highly virulent for mice, whereas most isolates of serotype 7 were weakly virulent. In swine, all isolates of serotype 2 were highly virulent, capable of inducing generalized urticarial lesions with depression and anorexia. On the other hand, 37 of 43 isolates of serotypes other than 2 induced no clinical signs, and the remaining 6 isolates induced local urticarial lesions at the site of inoculation in swine. The MIC of dihydrostreptomycin ranged from 1.56 to 100 micrograms/ml. All of the dihydrostreptomycin-resistant strains belonged to serotype 2. The high virulence of E. rhusiopathiae strains of serotype 2 harbored in the tonsils suggests a possible role of such strains in the cause of swine erysipelas. In contrast, members of the other nonvirulent or weakly virulent group, mainly serotype 7 strains, were considered to be resident in porcine tonsils.