We have presented a case of tracheobronchitis due to C pseudodiphtheriticum in a patient with COPD who was treated with prednisone and apparently was not otherwise immunocompromised. Chronic lung disease seems to predispose to infection with C pseudodiphtheriticum; it can also occur in the immunocompetent host. This organism, when isolated in pure culture, should not be dismissed as a contaminant, but must be considered a possible etiologic agent. Sensitivity of diphtheroids to antibiotics is extremely variable. We believe vancomycin should be used in respiratory tract infections caused by diphtheroids, including C pseudodiphtheriticum, until the results of in vitro antibiotic susceptibility tests are available.