Serologic, biochemical, and pathogenic characteristics of 11 porcine isolants of Erysipelothrix rhusiopathiae that could not be placed in any of 16 established serotypes, were examined. On the basis of double-diffusion percipitin reactions, isolants were divided into 4 serologic variants, given serotype designations 17, 18, 19, and 20. Biochemical activity of the isolants was typical of E rhusiopathiae. One or more isolants of each serologic variant were pathogenic for both mice and swine.