Forty-nine infections with the Malayan IV strain of Plasmodium falciparum were induced in different species of Aotus monkeys. The parasite was shown to be infective to four different species of Aotus monkeys via the inoculation of parasitized erythrocytes. Sporozoite transmission was obtained to A. lemurinus griseimembra, A. vociferans, and hybrid monkeys with A. azarae boliviensis × A. nancymai and A. lemurinus griseimembra × A. nancymai parentage. Anopheles freeborni and An. stephensi mosquitoes fed readily on animals and through membranes; both supported the development of infective sporozoites. Markedly increased levels of mosquito infection were routinely obtained by membrane feeding, indicating the presence of serum factors inhibitory to infection. The Malayan IV strain appears suitable for blood-stage and transmission-blocking vaccine trials.