The insular population of barren-ground caribou (Rangifer tarandus groenlandicus) on Coats Island, Northwest Territories, is ultimately limited by winter food resources. This study was undertaken to assess forage biomass available during summer and to determine the effects of snow on forage availability in winter. Vegetation was low in density and diversity, but provided ample forage of high quality in summer for the 2000-2500 caribou existing on the island. Aboveground green biomass averaged only 50-60 g.m^-2 in mesic and wet meadow, the most productive habitats. Caribou fed almost exclusively in meadows throughout the snow-free period; willows (Salix spp.) constituted 80-90% of rumen contents in caribou collected during the summer. Lichens were scarce and were a major food only at the beginning and end of winter. During mid and late winter, hard-packed snow averaged 4000-9000 g.cm^-2 in hardness and 50-80 cm in depth on all low-lying vegetation. Under these conditions, caribou cratered only where snow was less than 10-20 cm deep; consequently feeding was limited to the slopes of wind-blow ridges and the tops of high-centre polygons. Forage available under these winter conditions was drastically reduced in quantity and quality compared with forage available in summer. During winter 1983-1984, snow accumulated rapidly and prolonged deep snow cover was associated with high calf mortality.