The effects of experimental trampling due to hikers, horses and motorcycles were studied in a Festuca idahoensis-Poa pratensis meadow and in a Pinus albicaulis-Vaccinium scoparium forest. Horses and motorcycles were more damaging than hikers. Motorcycles were most damaging when going uphill while hikers and horses were most damaging when going downhill. Damage was less rapid in grassy vegetation than in shrubby vegetation. Damage was generally greater on slopes than on level ground.