Bacterial endocarditis due to Corynebacterium xerosis developed in a previously healthy person. Diphtheroid infection is a rare cause of endocarditis and, when present, it usually affects immunocompromised hosts or prosthetic valves. There are few reports of diphtheroid endocarditis on intact valves, and, to our knowledge, this is the first case in which the offending organism was identified as C xerosis. We call attention to the virulence of C xerosis in a person with no previous valvular disease.