Twenty-one splenectomized Aotus vociferans monkeys were infected with the different strains/clones of Plasmodium falciparum. Maximum parasitemia ranged from 1,302 to 1,460,000 parasites per mm3. Only the Santa Lucia strain was shown to produce gametocytes for extended periods. Gametocytes produced during the primary episode of parasitemia were highly infective to Anopheles freeborni mosquitoes. Gametocytes produced during recrudescence were not infective to mosquitoes feeding directly on the animals. This lack of mosquito infection during recrudescence periods suggests the presence of transmission-blocking immunity, which may be important in understanding the control of malaria through immunologic initiatives.