Flight and host-seeking activity of adult black flies were studied in the Souris River area, Manitoba. Six species, Simulium meridionale, Simulium luggeri, Simulium johannseni, Simulium venustrum/ verecundum complex and Simulium vittatum were potential pest species. These black flies fed on cattle, humans, poultry and horses. Simulium meridionale and Simulium luggeri are potentially the most important black fly pests in the area because they are abundant, they attack a wide range of hosts and they are multivoltine in the Souris River. Flight and feeding activity occurred between 0600 h and 2200 hr, at air temperatures between 17.3C and 36.7C, at relative humidities between 7% and 81% at light intensities between 0.6 and 11,250 lux, at winds with gusts up to 15.7 km/h and on days with 0-100% cloud cover. Most host-seeking adults were active between 1900 h and 2100 h, at air temperatures between 25C and 30C and at light intensities between 60 and 1,620 lux. Simulium meridionale, Simulium luggeri, Simulium johannseni Simulium venustum/verecundum complex are anautogenous while Simulium vittatum and Cnephia dacotensis are autogenous. Cone samplers did not correctly estimate larval populations, and were of little value in predicting adult population size. Cone samplers gave the relative stage of development and the relative proportions of black fly species present.