This communication concerns a case of endocarditis caused by Corynebacterium diphtheriae. The patient was a 35-year-old male drug addict who was brought to the hospital with fever, chills, and abdominal pain. Two days after admission, blood cultures were found to be growing gram-positive rods suggestive of diphtheroids. Repeated blood cultures grew the same organism, which was identified as a nontoxigenic strain of C. diphtheriae. The patient subsequently was identified as having acquired immune deficiency syndrome. Although isolates are divided into toxigenic and nontoxigenic strains, all isolates of C. diphtheriae should be considered potentially toxigenic. Because diphtheria generally is considered only of historic interest, few laboratories perform tests to identify it and instead report all isolates as "diphtheroids" or Corynebacterium. Because all isolates are potentially toxigenic, and because there is a large reservoir of nonimmunized people, laboratories must be alert to possible serious epidemiologic situations.