This report deals with the major features of untreated infections with Plasmodium inui as exhibited in a group of 31 rhesus monkeys (27 inoculated with trophozoites, four with sporozoites). Infections in nine monkeys were followed to self-cure. In 22 subjects they were interrupted prematurely; in 17 by chemotherapy 454 to 3,931 days after onset of patency; in 5 by death 491 to 1,025 days after onset of patency. Substantial monkey-to-monkey variations were encountered in the intensity of the parasitemia and its cyclic undulations, in the morbid features of the disease, and in the duration of infection. Thus initial peak parasitemias varied by as much as 15-fold. Some post-peak parasitemias were sustained at relatively constant and moderately high levels for many months; others fluctuated cyclically between barely detectable and readily countable numbers. In most subjects, there was no evidence of morbidity attributable to malarial infection; in two, reactions were severe enough to lead to death. The duration of untreated infections could be as brief as 14 months or could approach 14 years. With due allowance for premature interruptions, the majority of infections could be expected to persist for 4--6 years.