The clinical and laboratory findings of a spontaneous disease, resembling human leprosy, in a chimpanzee are described. The disease was a chronic progressive dermatitis characterized by nodular thickenings of the dermis and involving the ears, eyebrows, nostrils, and lips. A maculopapular rash was also present. Numerous acid-fast organisms were found in nasal swabs and in dermal lesions, including nerves. Attempts to culture acid-fast organisms in artificial media have failed. At this time, the only features of the etiologic agent of this disease that are inconsistent with those of Mycobacterium leprae are failure of the organisms to oxidize 3, 4-dihydroxyphenylalanine and failure to pyridine to remove the acid-fast staining property of the bacilli.