Deals with the ecology of mice and voles Peromyscus maniculatus, Clethrionomys gapperi and C. rutilus in the subnival environment, mainly on the Alberta plateau but with one plot and some traplines in the Mackenzie lowlands, both in the taiga zone where the snow cover is more uniform than in the tundra. Winters 1965-67 measurements of air and subnival (moss layer) temperatures, snow depths and densities, also changes in weight and numbers of species trapped are represented graphically. Mortality rates varied by species and year. The winter weight of all three species is less than in summer. Annual differences in mean body weight were small and not related to differences in population density.