The effect of lowered host-environmental temperature upon the development and maturation of the preerythrocytic tissue stages of rodent malaria parasites has been investigated in two strains of Plasmodium berghei originating from the highlands of Katanga. Young albino rats inoculated with massive sporozoite doses of P. berghei NK 65 and maintained for 48 hours at 12° C developed small, stunted tissue schizonts, averaging 11 × 15 microns, of a distinct morphology. Control rats kept at room temperature of 27° C showed mature tissue schizonts of normal growth averaging 24 × 29 microns. Blood from the rats kept at lower temperature for 48 to 50 hours failed to produce parasitemia when inoculated into susceptible recipient mice. All the mice given blood from control rats developed parasitemia. However, when sporozite-inoculated rats were kept for 96 hours or longer at 12° C they developed parasitemia and their liver showed maturation of 10% of the preerythrocytic schizonts. Experiments with the ANKA strain of P. berghei did not show significant differences in size and morphology between parasites in rats kept for 46.5 hours at 9° C and 12° C and those in controls kept at 20° C. However, subinoculation of blood from the low temperature experimental groups into recipient mice at 46.5 hours after intravenous sporozoite inoculation failed to produce parasitemia, whereas all the recipient mice from the control groups developed parasitemia in 4 or 5 days. The findings are discussed in the light of the evolution of plasmodia and the phenomena of relapse and delayed primary attack in certain malaria infections.