The effects of Leucocytozoon simondi on black ducks, mallards, and white Pekins were evaluated by comparing natural and experimental infections in immature and adult ducks. Higher mortality rates were recorded in immature white Pekins (75%) than in mallards (40%) and black ducks (18%) exposed continually to infected flies. Comparable doses of sporozoites inoculated into ducklings of the three species gave similar results, mortality in white Pekins was high (91%), moderate in mallards (58%), and black ducks (54%). Parasitemias, despite some variation, were higher in the endemic species. Lowest red blood cell volumes were noticed in infected white Pekins. Mortality of 60% was noted in adult white Pekins and deaths among adult mallards and black ducks were rare. Investigations of the spring relapse of L. simondi indicated more parasites in the peripheral blood of black ducks than in mallards and white Pekins. The endemic species appear more tolerant to the parasites.