Seven splenectomized chimpanzees were infected with the Nigerian I/CDC strain of Plasmodium ovale. Two of the animals had no history of previous malarial infection whereas three had been infected with P. vivax, one with P. malariae, and one with P. vivax and P. malariae. The two animals with no previous malarial experience had maximum parasitemias of 88,700 and 127,000 per mm3 while the other animals had maximum parasitemias ranging from 10,100 to 60,600 per mm3. Anopheles freeborni, An. dirus, An. stephensi, and An. gambiae were readily infected via membrane feeding on heparinized blood obtained from these chimpanzees during the ascending phases of their primary attacks. The parasitemias in the chimpanzees with previous malarial experience were transient.