An examination of the gametocytemias and mortalities in parallel groups of white pekin and muscovy ducklings shows that the two hosts developed very different types of infections. Following relatively low exposure risks which allowed at least some of the infected pekins to survive, fewer muscovies became patent, they developed lower parasitemias, and suffered fewer deaths. Even when transmission was severe enough so that all exposed ducklings of both species became patent and died, muscovy deaths occurred relatively late, during the period of declining parasitemia, whereas pekin deaths were
invariably associated with early fulminating parasitemia. Furthermore, muscovies appeared to show less correlation between parasitemia and the severity of disease than did pekins. Thus, the parasitemias in Group A muscovies, all
of which died, were not appreciably higher than those in muscovies of Group
B, in which only one of eight died