In seasonally snow-covered environments, larger mammalian species occurring above the snow have been shown to broaden diet breadth in winter, but below the snow in the subnivean space the relationship between snow cover and diet is quite different The Broad-toothed Rat Mastacomys fuscus, an Australian cold climate endemic, is a specialist herbivore. The diet of M. fiiscus in the alpine zone of the Snowy Mountains was studied over the snow-free period and in winter. M. fusais showed a preference for monocots over dicots throughout the year, however, there was a greater diversity and evenness of plant taxa in the diet in snow-free seasons. In winter, fewer species dominated the diet, reducing the evenness as well as the diversity. Because of lack of cover from predators in the snow-free period, M. fuscus is constrained to foraging in areas with shrub cover, where access to preferred grasses may be reduced. By contrast, in the subnivean space, because of the shelter provided by the cover of snow, M. fuscus can build temporary above-ground grass nests and concentrate its foraging in areas of grassland which, in summer, provide no cover from predators. The fact that it is able to collect an excess of preferred Poa spp. sufficient to build nests, and these, together with assorted piles of grass and grass cut to lie in runways are discarded after winter suggests that in winter food availability is not limited.