Erysipelothrix infections are caused by the bacterium Erysipelothrix rhusiopathiae. The infection may be localized or generalized and is probably more common than the literature indicates. Erysipelothrix rhusiopathiae is a Gram-positive rod that is easily decolorized and may appear to be Gram-negative. Granules similar to those seen in diphtheroids are often present. Erysipelothrix rhusiopathiae is catalase negative and inhibited by potassium tellurite, reactions which differentiate it from both Listeria monocytogenes and diphtheroids. Erysipelothrix rhusiopathiae can cause disease in many species of animals, but it is most important as the causative agent of swine erysipelas. The clinical signs of swine erysipelas can be divided into three groups: acute, subacute, and chronic. The administration of antibiotics, usually penicillin, provides the most effective treatment for Erysipelothrix infections in animals. Penicillin is the drug of choice for Erysipelothrix infections in human patients.