The effects of grazing by the Norwegian lemming (Lemmus lemmus) and reindeer (Rangifer tarandus) on biomass and vegetation structure in a mountain snowbed habitat were studied using several grazer-exclosures kept for 5 or 15 yr. During the 5-yr exclusion experiment, the total biomass of mosses in particular, the main winter-food of lemmings, increased within fenced areas as compared to open control areas. In the 15-yr exclosures, Polytrichaceae mosses still dominated, but graminoids (Anthoxanthum odoratum and Carex bigelowii) had also increased. In these longer-term exclosures, dead plant material, consisting of Polytrichaceae moss and graminoid necromass and other plant litter, had increased. By contrast, Kiaeria moss and lichens were absent or had low biomass after 15 yr of grazer exclosure, probably as a result of competitive exclusion. The results suggest that the plant biomass in such low productivity mountain snowbeds can be reduced to a low level by grazing. Grazing also determines the dominance relationships in the plant community and prevents accumulation of plant litter. The observed effects of grazing are inconsistent with hypotheses suggesting that herbivory is of little importance in low productivity environments, but rather support the exploitation ecosystems hypothesis predicting strong grazing impact in such low productivity environments.