Studies have been conducted over the past 20 years, mainly at the Seney National Wildlife Refuge in northern Michigan. Losses in goslings occur every year. Every fourth year these losses are severe, approaching 80% of the gosling crop. Lesser losses occur during the fifth year, in 1969 these losses were about 50%. In the two intervening years losses were in the range of 17 to 27%. It is assumed that the losses in these off-years are due to predation, weather, and other habitat-related factors. In the years of severe losses we attribute the cause largely to Leucocytozoon. Although most of the goslings sampled each year demonstrate a parasitemia with Leucocytozoon, in the years of severe losses the parasitemia is much higher and accompanying characteristic pathological findings are much in evidence. The main vector of Leucocytozoon in ducks in the area is Simulium rugglesi which has been implicated in the transmission in ducks in Wisconsin and Ontario as well as in Michigan. However, the losses in the geese occur in early June before S. ruggIesi makes its appearance in any numbers. Field and laboratory studies implicate S. innocens as the most important vector in early transmission to the geese. This species of fly is most abundant in late May when the geese are hatching. Adult geese have been observed to have a relapse of parasitemia when they arrive at the nesting area in late April and during the egg-laying period. Experiments on captive birds indicate relapse parasitemia can be increased by various stress factors. It is hyposthesized that unattached adults, which remain more aggressive during the nesting season, are the chief source of parasites for the vector blackflies.