Carex bigelowii–Racomitrium lanuginosum moss heath has high conservation value in Britain, being one of the most extensive near-natural habitats and also the preferred habitat of dotterel (Eudromias morinellus). This rare and attractive bird has declined in Britain in the past century, and loss of Racomitrium heath due to heavy sheep grazing and/or nitrogen deposition is probably responsible. Erection of snow fencing for a ski corridor across Carex–Racomitrium heath on Glas Maol, a mountain rising to 1068 m in the eastern Highlands, affected sheep (Ovis aries) usage, and so gave an opportunity to compare trends in botanical composition under different grazing intensities. We began monitoring in 1990, four years after the fence’s erection, and report trends up to 2002/03. Adjacent to the fencing (0–10 m away) sheep usage was much increased due to improved shelter, and C. bigelowii and R. lanuginosum declined, the latter sharply. Racomitrium cover was already reduced by a third in 1990, and fell by a further third over the next 12 years. Grass cover increased to nearly equal Carex cover 16 years after the fence erection. Dicranum fuscescens also spread but lichens declined. There was longer snow-lie near the fence, this being correlated with sheep usage despite somewhat different incidence, and logistic regression showed that for the 1990–1996/97 period Racomitrium loss was rather more closely related to snow-lie than to sheep pellet-group density, whereas Agrostis increase was highly significantly related to pellet-group density. Distant to the fence the composition of the Carex–Racomitrium heath changed little over 12 years of monitoring. Agrostis increased and C. bigelowii declined, both changes being significant but much smaller than adjacent to the fence. Also Polytrichum alpinum increased significantly and some lichens declined. For Racomitrium there was a fall of only 2.5% from its initial cover of 40% in 1990. Since the dung counts showed only a negligible reduction in sheep usage between plots at 13–15 and 43–45 m from the fence, the trends in composition recorded at positions 19–20 and 39–40 m from the fence apply to the extensive moss heath used by the dotterel on Glas Maol. These birds still nest in the distant zone, and we judge that the condition of the Carex–Racomitrium heath will remain satisfactory for them unless sheep usage increases by 25% or more. However, the ongoing loss of lichens and the sparsity of Vaccinium myrtillus imply that the current level of sheep grazing has appreciably modified this community from its former pristine condition.