Physiologically active thalli of the lichen Evernia mesomorpha Nyl. were very sensitive to short-term fumigations with low concentrations of gaseous sulphur dioxide. Net CO2 assimilation rate (NAR) was significantly reduced after exposure to 0.085 μl l−1 (250μg m−3) SO2 for 1 h or more, and the reduction increased with increasing concentration. Duration of exposure had no significant effect on NAR, indicating the importance of rate of SO2 uptake rather than the total amount absorbed. Respiration was significantly reduced after 4 h or more of exposure to 0.265 μl l−1 (639 μg m−3) SO2 or higher. Recovery of NAR after fumigation was dependent on both SO2 concentration and duration of fumigation, and on the time allowed for recovery. Virtually complete recovery occurred within 24 h after episodes with up to 0.355 μl l−1 (856 μg m−3) SO2 for 1 h and 0.085 μl l−1 SO2 for 4 h. Above these levels, recovery was incomplete or nonexistent after 24 h in clean air. The level of sensitivity found can be attributed to the environmental conditions during fumigation, which prevented thallus desiccation and inactivity. Based on this study, neither the concept of dose (concentration × time) nor that of ‘threshold’ levels of SO2 fumigations are supported. Peak exposures to SO2 for short periods may be of primary importance in determining the survival of lichens in industrial areas.