Polytrichum formosum Hedw. was exposed to 60 nl litre(-1) (122.4 microm(-3)) NO2 for 37 weeks in a closed chamber fumigation system. This concentration was chosen to simulate roadside levels in rural areas. Over an initial winter period (October-January) growth of existing shoots was stimulated by NO2. When new growth was recorded in April and May, NO2 pollution over winter and spring had resulted in a 36% reduction in new (< 1 cm) shoot production, and a 46% reduction in old shoots showing new growth. It is concluded that plants of Polytrichum formosum Hedw. growing near to roads may be adversely affected by NO2 pollution. Adverse effects of NO2 on plants and possible synergistic effects with other pollutants could cause growth reductions in sensitive species, thus affecting species composition of roadside vegetation.