White Pekin ducklings kept outdoors in Algonquin Park, Ontario, frequently become infected with Ornithofilaria fallisensis. Ducklings exposed during the black-fly seasons in 1952–1955 became infected. The microfilaria of O. fallisensis developed to the infective stage in Simulium venustum Say, S. parnassum Mall., S. rugglesi Nicholson and Mickel, and various members of the subgenus Eusimulium identified as S. euryadminiculum Davies, S. croxtoni Nicholson and Mickel, and S. latipes Meigen. Only the last four species were found feeding naturally on the ducks in 1955. The microfilaria did not develop significantly in in several species of mosquitoes nor in Culicoides sp. Members of the subgenus Eusimulium were collected from ducks in 1955 only during the early part of the black-fly season whereas S. rugglesi was collected from May 26 to the middle of July. Ducklings exposed when these ornithophilic simulüds were active became infected and a consideration of the times after exposure at which microfilariae appeared in their blood indicates that members of Eusimulium were the vectors of O. fallisensis in the first part of the fly season and that S. rugglesi was the sole vector thereafter. The development of the microfilaria to the infective stage takes place in the haemocoele of the black fly and requires 7–14 days depending on the temperature. There are two molts during development and a "sausage-stage". Microfilariae appeared in the blood of three parasite-free ducklings 30–36 days after third-stage larvae were injected into them; adult worms were recovered from one of these birds. The microfilariae of O. fallisensis exhibit a diurnal periodicity. The courses of the microfilaremias of experimentally and naturally infected birds were compared. Re-exposure failed to alter the declining microfilaremias of ducks or to result in a reappearance of microfilariae in ducks from whose blood microfilariae had disappeared. Since parasite-free, adult ducks became infected during exposure, it is concluded that infected birds acquire an immunity to O. fallisensis. A few ducklings seemed to exhibit a natural resistance to infection. Adult O. fallisensis were found in a black duck (Anas rubripes) from Algonquin Park and it is suggested that these birds are the reservoirs of infection in the Park.