The entomopathogenic fungus, Beauveria bassiana, was isolated from pulmonary lesions of a dead American alligator (Alligator mississipiensis) at the Oklahoma City Zoo. Colonies of the fungus, which had sporulated in vivo, were found in the thoracic air spaces. Septate, branching hyphae and fungal spores were seen in stained histologic sections of pleura and lung. Dissemination to other viscera had not occurred. This case indicated that B bassiana, a rare vertebrate pathogen, may be a fatal mycotic agent in captive reptiles.