There is increasing evidence from behavioral and chemical ecology that non-nutritional resources have a significant effect on the health of wild animal populations, both through deliberate self-medication (zoopharmacognosy) and dietary prophylaxis. This dissertation reports the first systematic investigation of anti-parasitic self-medication in Yakushima Japanese macaques (Macaca fuscata yakui). Adult female macaques of NA2 group on Yakushima were studied for one full seasonal cycle, their health measured through fecal parasitological analysis, and their foraging behavior recorded through focal observation. A set of candidate medicinal plants and clay were selected from literature and preliminary observations, and interactions with these recorded in continuous detail. Statistical analyses were designed to reveal patterns in parasite load, parasitological stimuli for self-medicative behavior and the functional efficacy of those behaviors. Ethnographic interviews were conducted with several practitioners of traditional medicine in Japan, and their knowledge compared with the macaque results.The macaques were found to be infected with five species of gastro-intestinal parasite, but appear to tolerate the infections with few symptoms. The coarse grass Miscanthus sinensis was found to be associated with severe Streptopharagus pigmentatus infection, and may act to physically expel this nematode taxon. Geophagy (clay eating) is practiced year-round, but while it may account for the low incidence of diarrhea, it was not found to be directly associated with parasite infection. Instead it appears to function in the detoxification of invertebrate and plant foods. Records of juvenile observation of their parents suggest that these two behaviors may be a cultural tradition in the population. The ingestion of several other plants seems to incorporate a range of pharmacologically active phytochemicals into the macaque diet. While some are rare and deserve further investigation, others are utilized as food items and any health benefit accrued incidentally to this. Interview data reveal that the traditional pharmacopoeia of Japan differs significantly from that available to the macaques, and is proposed to be due to ecological differences between Yakushima and the centers of traditional medicine in Japan. The significance of these results is discussed with regard to conservation medicine, and theories of the emergence of human medicine.