Four gibbons were inoculated subcutaneously with larvae of Dirofilaria immitis and studied for periods of time up to 14 months. Approximately 2 months following inoculation, skin test reactions converted to positive, eosinophil counts rose, and serological tests were positive for Dirofilaria-reactive antibody. Generally, these laboratory manifestations of infection persisted for the remainder of the study, but microfilaremia was never detected. Both live and dead heartworms were found in the heart and lungs of the infected animals. The pulmonary lesions associated with the infection closely resembled those reported in dogs.