The primary objective of this project was to study the life cycle and ecology of Plasmodium pitheci, a malaria parasite of the orang-utan. The field work was based on the orang-utan rehabilitation centre in the Sepilok Forest Reserve of eastern Sabah. Two visits were made to Sepilok, the first in February and March, 1972, and the second (by W.P.) in January 1974. On the first visit two species of 'surrogate host' were taken to Sabah, i.e. chimpanzees and Aotus monkeys for experimental work. The arboreal habitat of the orang-utan in the dipterocarp forests of eastern Sabah is described. In the Sepilok Forest Reserve dwell gibbons and leaf-monkeys, in addition to a small population of semi-domesticated and wild, free-ranging orang-utans of various ages. Although numerous species of anopheline mosquitoes have been collected in eastern Sabah, longitudinal studies are not available. Anopheles balabacensis was caught both attracted to orang-utans and to man at Sepilok. This species which is the main vector of human malaria in the north of Borneo, is suspected also of transmitting orang-utan malaria in this part of Sabah. Repeated blood examinations have been made on a number of orang-utans in the centre since 1966 and a high prevalence of infection was recorded with Plasmodium pitheci. In 1966 10 out of 19 animals had demonstrable parasitaemia. Detailed case histories are presented to show the course of parasitaemia in several orang-utans. Infections of P. pitheci were found to run a very chronic course. During the 1972 expedition a second, previously undescribed malaria parasite of the orang-utan was discovered, and was named P. silvaticum. The new parasite was successfully transmitted both by blood inoculation and, later, by sporozoite inoculation, into splenectomized chimpanzees. Although both species of malaria parasite may cause transitory signs of illness, orang-utans in general appear to be little discomforted by the infection. The animals do however suffer from other infectious diseases such as amoebic and balantidial dysentery, and melioidosis is a serious natural hazard which may have accounted for several deaths of wild orang-utans. An unidentified, intraerythrocytic structure that appeared in the blood of one chimpanzee, which had been inoculated with blood from an orang-utan, may have contributed to its death. Detailed descriptions and illustrations of P. pitheci and P. silvaticum are given. All stages of the life cycle of P. silvaticum are known (the tissue stages having been described in the liver of a 'surrogate host', the chimpanzee) but only the blood and sporogonic stages of P. pitheci have been seen. This species was not infective to a chimpanzee, although there is an earlier report of a transient infection in this host by other workers. In the blood both parasites showed a tertian periodicity. From the appearance of the tissue schizonts on the seventh day it was estimated that the complete pre-erythrocytic cycle of P. silvaticum in the chimpanzee would occupy 8 days. P. pitheci is readily distinguished from P. silvaticum, and most closely resembles P. hylobati and P. youngi of the gibbon. The sporogony of both orang-utan parasites in anopheline mosquitoes is described. Although P. pitheci produced a transient parasitaemia in the splenectomized gibbon following blood inoculation, P. silvaticum did not do so, and neither parasite was infective to Aotus or Macaca. P. pitheci was also non-infective to man. P. silvaticum clearly belongs to the vivax-cynomolgi group and resembles P. eylesi of the gibbon. Several morphological and other features distinguish it from P. vivax with which a close comparison was made. Based on the new knowledge acquired of the two malaria parasites of the orang-utan and existing knowledge of other primate malarias, a new plan of the evolution of the primate malarias in relation to their hosts is proposed. This agrees well with other evidence linking the orang-utan with the gibbons, rather than with the other anthropoid apes, Pan, Gorilla and Homo. In considering the epizootiology of orang-utan malaria attention is drawn to the high prevalence of infection in relation to the relatively solitary habits and low population density of these apes. The hypotheses that P. silvaticum may produce zoonotic infections in man or, alternatively, that orang-utans may contract P. vivax malaria are considered, and rejected. The logistics of using exotic animals in this investigation are reviewed, and an outline of the complex technical procedures followed are provided in the appendix.