Aspects of the distribution, behavior, and mortality of pronghorns (Antilocapra americana) were documented in southeastern Alberta during the severe winter of 1977-78. As an apparent response to record snowfall in portions of their range, only 34% of the pronghorns observed during aerial surveys in January and February occupied typical winter ranges characterized by open basins with silver sagebrush (Artemisia cana). Many herds selected sites with reduced snow accumulation. Estimated mortality among the approximately 14,360 pronghorns entering the 1977-78 winter was 48.5%. Necropsies revealed significant losses in body weight, kidney fat, and femoral fat content of carcasses when compared with values for animals sampled before winter. Fawns of both sexes and adult males were more susceptible to winterkill than were adult females. Mummification and desiccation were evident in 68 of 82 fetuses examined. Potential adaptive values of the observed behavioral and physiological phenomena are discussed. J. WILDL. MANAGE. 46(4):991-1002 In Alberta, periodic severe winters have long been recognized as a major source of mortality for pronghorns (Wishart 1970). Typical of animals at the northern limits of their range, the physical features of pronghorns in Alberta appear to conform with both Allen's and Bergmann's rules (Mitchell 1971). Further behavioral adaptations for survival of pronghorns in winter have been described by Bruns (1977). Prolonged heavy snowfall and lack of access to woody browse have caused heavy mortality among pronghorns (Martinka 1967, Compton 1970, McKenzie 1970, West 1970). Some abnormal movements in Alberta and deaths attributable to exposure and malnutrition were reported by Mitchell (1965, 1980) following the 1964-65 winter. Pronghorns in Alberta use widely different ranges throughout the year but most occupy specific wintering areas (Barrett and Vriend 1980). Preferred winter ranges contain an average of 88% native vegetation consisting primarily of grasslands with varying densities of silver sagebrush cover (Barrett 1980). Previous accounts of extensive winter losses of pronghorns have provided little quantitative information on many facets of die-offs. This paper documents the abnormal distribution of pronghorns, extent of mortality, necropsy results, and general response to supplemental feed during a severe winter in Alberta. I acknowledge the technical assistance of J. Clark, L. Gudmundson, and H. Vriend of the Alberta Fish and Wildlife Division, Lethbridge. The laboratory assistance of N. Holfeld of the Veterinary Services Division, Alberta Agriculture, Lethbridge is appreciated. The cooperation of several clubs of The Alberta Fish and Game Association, Alberta Fish and Wildlife Division Officers, and concerned landowners is acknowledged. J. Gunson, Alberta Fish and Wildlife Division, G. Chalmers, Veterinary Services Division, Alberta Agriculture, and A. Hawley, Alberta Environmental Centre, reviewed the manuscript. STUDY AREA AND METHODS