A case of prepatellar bursitis which, clinically, appeared to be a simple pyogenic infection proved to be an infection with Mycobacterium marinum (balnei). The bacteriology and epidemiology of this acid-fast bacillus is presented. It was one of fifty mycobacterial strains obtained from bone, joint, or other extrapulmonary lesions and cultured at the Mycobacterial Research Laboratory, Veterans Administration Hospital, Salt Lake City, Utah. Each of the fifty strains was readily classified as one of nine species or groups. It is important to search not only for tubercle bacilli but also for other acid-fast species in infections of musculoskeletal and soft tissues.