Uniformly fatal simian malaria was induced in ten rhesus monkeys by injection of Plasmodium knowlesi. The results of serial studies of platelet and blood coagulation factor levels suggested the occurrence of intravascular coagulation during the last 48 hours of the disease, concurrent with a marked fall in hematocrit levels. Fibrinogen survival was slightly decreased (two animals), but quantitative fibrinogen levels were elevated. Pathologic studies revealed only minimal evidence of fibrin deposition without indication of resultant tissue damage. The results are consistent with terminal intravascular coagulation possibly triggered by massive destruction of parasitized red blood cells.