Nine splenectomized chimpanzees were infected with the Uganda I/CDC strain of Plasmodium malariae. Two had no history of previous malarial infection, whereas 6 had been infected with P. vivax and 1 with P. vivax and P. ovale. The animals with no previous infection had maximum parasitemias of 8,740 and 10,800/mm3. The other animals had maximum parasite counts of 930–75,700/mm3. Anopheles freeborni, An. stephensi, An. dirus, An. maculatus, An. quadrimaculatus, An. culicifacies, An. arabiensis, and An. gambiae were readily infected by feeding through membranes on heparinized blood from these animals.