Confirmation of the existence of a persistent, uninucleate, dormant pre-erythrocytic stage, the hypnozoite, of the relapsing simian malaria parasite, Plasmodium cynomolgi bastianellii, has been obtained by means of experiments involving the intravenous injection into susceptible monkeys of 48 to 85 x 10 (6) sporozoites derived from mosquitoes of a different species and source than employed previously. The development of these hypnozoites was traced from 3 days until 105 days after sporozoite inoculation, employing a sensitive immunofluorescence technique followed by restaining with Giemsa. From an average mean diameter of 4 micrometers at 3 and 5 days, uninucleate hypnozoites grow to 5 micrometers at 7 days, then persist with little change until at least 105 days after infection. Strong evidence for the viability of these persistent forms was obtained by treatment of a host monkey with primaquine, which eliminated all trace of hypnozoites present 2 weeks before. Examination of hepatic tissue from a monkey injected with sporozoites 36 and 40 hours earlier revealed rare uninucleate pre-erythrocytic forms of 2.5-micrometers diameter. These early forms were present in hepatocytes in a density only approximately 1/30th of that expected on the basis of numbers of pre-erythrocytic stages found in the same animal's liver 7 days after infection. Nevertheless, subinoculation experiments appeared to rule out the circulation as a vehicle for dissemination of any putative early intermediate hepatotropic forms from another site.