Nine Saimiri sciureus boliviensis monkeys were inoculated with sporozoites of Plasmodium vivax (Chesson strain) dissected from Anopheles stephensi mosquitoes infected by feeding on blood from infected chimpanzees. The animals were splenectomized 7 days after inoculation. Seven animals developed infections with prepatent periods ranging from 12 to 43 days (mean of 19.6 days). Parasitemias were low during the first 50 days. Maximum parasitemias in 5 animals in which the strain adapted ranged from 10,000 to 46,800 per mm3. Anopheles freeborni mosquitoes were infected by feeding on 4 of the monkeys.