A 6-year retrospective study of necropsy material from a colony of 200 gibbons showed strongyloidiasis to be the most frequent cause of death (24 cases). Clinical signs included diarrhea, constipation, weight loss, paralytic ileus and dyspnea. Lesions were most frequent in the gastrointestinal tract and lungs. In the gut there were erosive and ulcerative enteritis associated with adult female parasites and rhabditiform larvae and acute and granulomatous enterocolitis associated with invading filariform larvae. There was severe multifocal or diffuse hemorrhage associated with migrating larvae in the lungs of 23 gibbons. Filariform larvae and the lesions they caused also occurred in various tissues; this was commensurate with the wide distribution of these larvae when hyperinfection occurred.