Humans are the most important reservoir of human malaria, except as regards P. malariae, which is common to man, the African apes and probably some South American monkeys. Non-human primates are naturally infected by malaria parasite species, some of which are closely related to the human malarias, and which therefore can infect humans experimentally. Natural transmission of these non-human primate malarias to humans occurs sporadically. Recently, P. knowlesi, a parasite of Old World monkeys, has been documented as a cause of hundreds of human infections and some fatalities in Malaysia. Investigations are ongoing to determine the extent of transmission to humans; it is suggested that non-human primates may be a more important source for malaria in humans in certain geographical situations than previously thought.