The purpose of this study was to clarify the taxonomic relationship between all the serovars and species of the genus Erysipelothrix by performing DNA-DNA hybridization experiments, the customary criterion for separation of bacterial genospecies. A total of 93 strains were isolated from a wide variety of sources, including pigs affected with acute or chronic erysipelas, other diseased animals, healthy animals, fish, retail meats, and environmental materials from throughout the world during the period 1958 to 1996. The present data on phenotypic characterization and DNA relatedness values demonstrate that 24 strains (96%) of E. tonsillarum are avirulent for swine, whereas 39 strains (66%) of genomic E. rhusiopathiae induced generalized or local urticarial lesion in swine after intradermal inoculation. This observation suggests that genomic E. tonsillarum has little etiological significance. Three minor groups contained several strains which exhibited minimal association with each type strain of E. rhusiopathiae and E. tonsillarum. In conclusion, it was confirmed that members of the E. rhusiopathiae and E. tonsillarum groups resemble each other in regard to many phenotypic characteristics, but differ in their ability to produce acid from saccharose and in their pathogenicity for swine. The genus Erysipelothrix certainly contains two main species: E. rhusiopathiae and E. tonsillarum.