Human hosts exposed to infection are model systems for studying the interactions of parasites with each other and with their environments. This paper uses published epidemiological data to demonstrate an interaction among the species of human malaria that is expected from ecological and evolutionary theory. Under certain circumstances, there are fewer mixed malarial infections in human beings than would be expected if infection with one species of malaria were independent of infection with each other species. This reduction in the number of mixed infections is strongly associated with enlargement of the spleen of the human hosts, and less strongly with situations that stimulate immune responses. Such heterologous resistance is probably best explained as a partial heterologous immunity to malaria in man, since experiments in other mammals have shown that immune mechanisms can eliminate or reduce the level of mixed infections. Though competition among mixed malaria species for nutrients in limited supply in...