The ornithophilic Simuliidae that feed on waterfowl were studied in northern Michigan. With the exception of one Cnephia sp., only Simulium rugglesi (Nicholson and Mickel) were collected attacking birds in Michigan during the study period from 15 April 1959 to 21 August 1962. The number of flies trapped in Michigan was never comparable to similar collections in Canada and Wisconsin. Nevertheless, a high incidence of Leucocytozoon infection was observed in both Cheboygan and Presque Isle Counties near streams, in contrast to substantially lower incidences of infection and fatality on the lake front at the UMBS and at Riggsville (both in Cheboygan Co. and both away from streams). Host preference studies were made by exposing mallard ducks and 14 other species of birds. New host feeding records for S. rugglesi included black terns, herring gulls, black crown night herons, sandpipers, robin fledglings, and blackbird fledglings. S. rugglesi preferred ducks over all other birds except gulls and blackbirds, colored ducks and ducklings over white ducklings and over goslings of all colors. They preferred colored ducklings and large white ducklings over goslings. S. rugglesi transmits Leucocytozoon among summer resident birds rather than the birds migrating through this region of Michigan. Epizootic conditions here are therefore significantly different from those in areas with larger and earlier appearing S. rugglesi populations farther north.