Three successive summer surveys in northern Michigan disclosed that Leucocytozoon simondi infections among farm-raised ducks were comparatively uncommon late in June or early in July. However, surveys conducted late in July or early in August demonstrated that very few ducks escaped infection during the intervening weeks. In order that the epizootic might be more closely studied, successive test groups of young ducklings (6 weeks of age) were exposed to natural infection under ordinary farm conditions for 8-day periods during the summer of 1950. Data derived from the study of each of these groups of birds after their removal to screened quarters yielded the following results: (1) Ducks exposed in late June or early July did not become infected rapidly, nor did all birds become infected; of those that did acquire infections none died. (2) In birds of 3 successive test groups exposed between July 11 and August 4, infections were very rapidly acquired; group attack rates ranged from 90 to 100 per cent and the case fatality rates by group varied from 14 to 83 per cent. (3) By mid-August the epizootic had so declined that it was no longer measurable by the technique employed; at this time, none of a test group of 10 birds acquired infection as a result of farm exposure. The birds introduced as transients for brief periods during the course of the 1949 and 1950 epizootic seasons suffered 3 to 5 times the mortality rate of the indigenous farm birds, despite the fact that the latter were exposed to. infection during the entire summer. It is suggested that the indigenous birds, having been exposed very early in the summer, received mild infections which rendered them immune to the heavier, mortality-producing infections reflected in the data for the transient groups during mid-summer. In terms of flock management, the evidence seems to warrant the conclusion that if young birds are introduced sufficiently early in the season (before the end of June) they would probably survive the epizootic although they would almost certainly become infected. In the case of these young birds the protection against fatal issue resulting from infection would seem to lie in an acquired immunity rather than in one accountable to age. However, considerably older birds (18 weeks of age) do seem to enjoy greater chances of surviving primary infections acquired during the midepizootic season. The sex of the host appears to be of no consequence in susceptibility to L. simondi infection, nor does it mediate in the outcome of these infections. In the light of the newly available epizootiological data, several questions are raised regarding that aspect of the problem which bears on the role and identity of the arthropod vector.