During an irruption of Snowy Owls (Bubo scandiaca) into western Montana in the winter of 2005–2006, we collected dietary data from 42 to 48 owls from three sites. We recorded 5400 prey from 1313 pellets, pellet fragments, and prey remains. Voles represented >90% of the prey from each site. Montane voles (Microtus montanus) dominated at two sites and meadow voles (M. pennsylvanicus) dominated at the third site. Few other prey species were eaten. Food-niche breadth was 1.14, 1.41, 1.92, and dietary evenness was 0.377, 0.398, 0.584 for the three Montana sites. We compared these data to those from six other North American studies reporting 1208 prey items collectively. A random sample of 100 pellets yielded a mean dry mass of 19 g, and a mean length × width of 82 mm × 30 mm, and the number of prey per pellet averaged 5 (SD ± 2). Mean body mass of voles eaten by the owls was 30 g. During the following winter (2006–2007), only three Snowy Owls were found in the same study areas.