Non-human primates play host to a broad spectrum of parasites and commensals. The range of organisms present and frequently the intensity of infection is dependent upon or related to biologic circumstances responsible for parasite transmission and propagation. Most of the protozoans of primates are considered as commensals with limited pathogenic potentials. Some protozoa ( Entamoeba and Balantidium ), however, are responsible for variable pathologic involvement, especially in the intestinal tract. Others ( Babesia, Plasmodium and Trypanosoma ) may induce subtle or transient infections of medical concern The helminths, by contrast, are represented by a formidable list of worms responsible for tissue destruction and macroscopic pathology in the intestine as well as in other pertinent viscera. Bertiella may be presented in a number of African primates but adult cestodes as a rule cause little disease. Larval cestodes ( Echinococcus and Multiceps ), however, may cause impressive involvement of viscera and frequently are found in the body cavity. Although their presence may not be lethal, different nematodes, e.g. Molineus and acanthocephalans may constitute serious intrusion and may be common in South American primates and Oesophagostomum may be prevalent in some African populations. The threat of Strongyloides in chimpanzees is well recognized. Macroscopic filariae may be resident in connective tissues or lungs and microfilariae are found in the circulatory system. Somatic infections may result from larval pentastomes in various viscera. Possible or even probable synergism of non-pathogenic parasite infection in combination with commensals or microbiologic organisms is poorly understood and offers opportunity for investigation. In general there seems to be a fairly well established equilibrium between primates and their parasites.