The primary data in this report showed that prolonged multiplication of the M strain of Plasmodium cynomolgi in splenectomized rhesus monkeys led frequently to emergence of parasites whose virulence for monkeys with intact spleens was markedly attenuated while that for splenectomized monkeys was unimpaired. Expressions of attenuation regularly included reductions in the height of the peak parasitemia and in some instances failure to induce infection with inocula 1,000-fold those infective for splenectomized monkeys. Attenuation of virulence, once established, was maintained through as many as 17 serial passages in intact monkeys and more than 100 such transfers in splenectomized monkeys. Not only asexual blood stages, but also sexual stages (gametocytes) carried the attenuated characteristic, as indicated by its ready passage through mosquitoes. Studies in monkeys with sporozoite-induced infections showed that the persisting exoerythrocytic stages did not participate in the attenuation phenomenon; for when erythrocytic parasites of reduced virulence were cleared from the blood by either immune processes or chemotherapy, they were replaced upon relapse with parasites of unimpaired virulence. A major effort to determine whether splenectomized monkeys carried plasma or erythrocyte factors responsible for displays of attenuated virulence was nonproductive. An intact spleen was the essential element in these displays, for they disappeared immediately upon removal of this organ. The attenuation phenomenon probably reflects selection of spontaneously occurring mutants with limited capacity to escape normal splenic clearance mechanisms and unimpaired or even enhanced capacity to multiply in splenectomized monkeys.