Leucocytozoon simondi is a serious pathogen of both wild and domestic ducks and has been associated with severe and costly epizootics in northern Michigan and in Canada. The parasite is closely related to the Plasmodium of malaria, and, in general, its life cycle is quite similar to that of the malaria organism. The cycle of L. simondi differs from that of the plasmodia in two major respects: (1) L. simondi is transmitted by blackflies (Simuliidae) rather than by mosquitoes, and the development in the fly, according to O’Roke (1934), requires much less time that does that of the Plasmodium in the mosquito; and (2) the asexual development of L. simondi in the vertebrate host appears to be entirely confined to cells of the internal organs, and only the sexual elements (gametocytes) are found in the circulating blood. A complete cycle from fly to duck and back can take place in a period as short as two weeks. Ducks usually retain their infections for long periods of time, and this, together with the spring relapse phenomenon, accounts for the survival of the parasite from one season of transmission to the next. Since there is available but little detailed information concerning the infection, this investigation was undertaken in order (1) to learn more of the epizootiology of L. simondi infections, (2) to elucidate the course of parasitemia during the primary attack, and (3) to study the nature of, and the circumstances surrounding, the phenomenon or relapse.