For many years, the black fly Simulium venustum Say has been considered to be the chief vector of Leucocytozoon simondi M. & L. (=L. anatis Wick.), a protozoan parasite infesting the bloodstream of ducks and geese and causing a malarialike disease that is usually fatal to immature birds. The organism undergoes part of its life-cycle in the stomach of the fly, which transmits it, through its bite, to the primary host. Its development in the latter results in the rupture of blood cells, with extensive destruction of tissue of the spleen, liver, and other organs. Although, superficially, the case against S. venustum appears to be already proved, recent studies of this species in Eastern Canada have indicated that some aspects of its supposed relation to L. simondi cannot readily be explained on the basis of what is known of its life-history and habits. My attention was focussed on this problem when I discovered in 1953 that the Simulium species reported by Walker (1927) as causing wholesale deaths in a flock of goslings at Fredericton, N.B., was neither Simulium bracteatum Coq. (a synonym of S. (Eusimulium) aureum Fries), as reported by Walker, nor S. venustum Say, as suggested by Twinn (1933).