Domestic sheep (Ovis aries) that graze in mountainous areas have the potential to affect Rocky Mountain bighorn sheep (Ovis canadensis canadensis) populations. Past research has identified domestic sheep as a potential vector for transmission of pneumophilic bacteria to bighorn sheep and several bighorn sheep population die-offs have been attributed to contact with domestic sheep. However, bighorn die-offs have also occurred in areas free of domestic sheep influence and some bighorn sheep populations have suffered no die-offs after coming in direct contact with domestic sheep. A great deal of research has been conducted historically on the demographics of bighorn sheep and their response to immunological stressors, such as Pasteurella spp. Additionally, a host of information exists in the literature that describes characteristics of habitat used most readily by Rocky Mountain bighorn sheep. However, little information exists to explain how habitat differences between populations contribute to the long-term success or failure of bighorn sheep populations. The purpose of this 2-year study is to evaluate the differences in spatial, vegetative, and geographic habitat characteristics of summer and winter ranges between historically stable and historically less stable bighorn sheep populations that occupy rangeland and open forest habitats in Montana in the presence of domestic sheep. Habitat variables will be quantified for two summers in the summer and winter ranges of two bighorn sheep populations in Montana, in both their entire seasonal ranges and areas identified as foraging habitat. Land cover, slope, aspect, elevation, solar radiation index (SRI), and distance to escape terrain will be quantified in each habitat using a Geographic Information System (GIS). Field sampling will be used to determine shrub canopy cover, frequency of graminoids and forbs, horizontal visibility, and soil nutrients of each habitat. Habitat characteristics of historically stable and less stable populations will be compared. Preliminary results will be presented.