Although only a small percentage of patients with prosthetic heart valves develop postoperative endocarditis, this infection is always severe and often fatal. The tissue adjacent to the inserted foreign body or the sutures may become a target for a wide variety of bacteria or fungi which are under ordinary circumstances of low pathogenicity. In this report we present a patient with prosthetic valve endocarditis due to Mycobacterium chelonei. The pathogenicity of this organism is still poorly defined. In the competent host it is capable of causing localized, indolent abscesses, usually on the site of contaminated injections. In the compromised patient this organism may be far more aggressive than has previously been recognized.