Nine new records of black flies bring the total number of species found in northern Manitoba to 21. Observations on the bionomics, especially of Simulium venustum. Say, are given. The attachment of black-fly larvae is determined by a mechanical factor, probably the water velocity gradient, perhaps conditioned by visual stimuli. Young larvae were heavily preyed upon by Hydra sp. and adults by the white crowned sparrow Zonotrichia leucophrys. Mermithid worms persisted as parasites in the pupal and adult stages. A method is described by which specimens of several species were reared individually in the field from half-grown larvae to the adults, and the exuviae retained. A number of laboratory rearing techniques were tried for S. venustum but few larvae molted from the first to the second instar. Adults did not feed on blood or lay eggs in the laboratory, and there was no evidence of successful mating. The difficulties in laboratory rearing of larvae are thought to be due to a sensitivity to autointoxication. The energy relationships of black flies are discussed briefly.