Comparative gametocytemias and mortality rates are reported for white pekin and muscovy ducklings simultaneously exposed to natural infections with Leucocytozoon simondi. Although both hosts became readily infected, mortalities and gametocyte levels differed considerably among the various exposure groups. Muscovies, however, consistently had much lower gametocytemias than pekins, tended to develop gametocytemia later than the pekins, and had proportionately fewer elongate forms (never more than five percent of the total). Furthermore, the mortality rate was lower among the muscovies and deaths were delayed. It is not clear whether such differences between the infections in pekin and muscovy ducklings result from differences in susceptibility to L. simondi or differences in susceptibility to attack by black flies. In any event, deaths among the muscovies appear to result from exoerythrocytic schizogony rather than from blood destruction.