Plasmodium fragile and the Macaca mulatta monkey are presented as a model system for the study of malarial vaccines. Four animals were immunized with culture-grown P. fragile merozoites and subsequently challenged with culture-produced parasites. One animal failed to develop a detectable parasitemia following primary challenge. Two other immunized animals had primary infections which were short-term. The parasitemias in these three monkeys following secondary challenge were short-term and the immunity was apparently sterilizing. The fourth immunized monkey had recrudescences of the primary and secondary infection but differed markedly from the four control animals. Indirect fluorescent antibody titers increased as a result of the immunization and were indicative of the level of immunity. Because of the many similarities to the human-P. falciparum model system, the P. fragile-M. mulatta system appears to be particularly well suited for a number of malaria vaccine studies.