Ducks exposed outdoors in Algonquin Park during the summer became infected with Leucocytozoon simondi and many of them died from the infection in June and July when black flies were abundant. The minimum prepatent period was five and a half days. Young parasites were observed in erythrocytes and lymphocytes; mature gametocytes, as shown by exflagellation of microgametocytes, occurred in round and elongated host cells. Asexual development was observed in the spleen, liver, heart, brain, lung, lymphoid tissue, and pancreas. The schizonts were large and contained, at maturity, more than a million merozoites about 1 μ in diameter. Sexual development of the parasites within two species of black flies was completed in three days at summer temperature. Ookinetes were present in the stomach of black flies about four to six hours after the flies had ingested blood containing gametocytes. Developing oocysts were found free among the stomach contents of flies 40–48 hr. after the blood meal. Sporozoites were observed between 60–70 hr. after the flies had ingested gametocytes. Artificial infections were produced in ducks following the injection of macerated black flies that had fed two and one-half to seven days previously on infected ducks. The resulting infections were less severe than those that resulted from natural infections, although the pattern was similar and a minimum prepatent period of eight days was observed. The asexual cycle was completed in fewer than six days although some asynchronicity was apparent. Artificial infections were produced following the injections of blood, spleen, liver, lung, and bone marrow that were taken from ducks at various intervals following their exposure to natural infection. The minimum prepatent period in these infections was eight days and low parasitemias were produced. Gametocytes survived for at least one week in peripheral blood. Ducks developed some resistance following repeated infections but single infections did not protect ducks that were exposed to infection six weeks later. Heavy infections developed in ducks that were splenectomized. The leucocytosis and anemia associated with infections were measured and recorded.