Three levels of scabbing damage to horses' ears, slight, moderate, and intense, were selected to assess the effects of black fly feeding on horses in the field. One of these categories was assigned to each ear examined. The numbers of black flies present in the ears were recorded. The numbers of flies found feeding (95% confid. inter.) and associated levels were slight, 3.1-4.5; moderate, 11.3-14.1; intense, 20.3-23.5. Simultaneous feeding by 14 or more flies per ear was accompanied by adverse behavior of the animal because of the resulting sores. The percent surface scabbing (mean + s. d.), measured from 35 mm color slides of representative ears, was: slight, 4.3 + 1.7; moderate, 10.8 + 4.2; and intense, 24.5 + 4.6. The blood meal volume for Simulium vittatum females was determined to be 2.5 + 0.3 mm.3