AbstractAlthough ozone is generally accepted to be an important phytotoxic air pollutant little is known of its effects on natural and semi-natural vegetation. In this study four species of Sphagnum, common to upland areas, (S. capillijolium(Ehrh.) Hedw.,S. cuspidatumHoffm.,S. papillosumLindb. andS. recurvumP. Beauv.) were exposed to acute ozone episodes,ie. 150 ppb (nl 1-1), for 6 hours at low temperature (5°C), in controlled chambers. The fumigation protocol caused a significant reduction in photosynthesis of treated plants in comparison to control plants for one species,S. recurvum, as measured by CO2assimilation and chlorophyll fluorescence. Membrane leakiness, measured by efflux of potassium ions, was increased. The three other species studied,S. capillijolium, S. cuspidatumandS. papillosumwere not sensitive to the ozone exposure. ForS. recurvumthe ratio of variable to maximal fluorescence (Fv/Fm ratio) was significantly reduced following ozone fumigation at 150 ppb (by an average of 21%). CO2assimilation of ozone treatedS. recurvumwas also significantly reduced in comparison to the control plants by approximately 50%.S. recurvumalso showed significantly increased membrane leakiness following treatment with ozone. Bryophytes by the very nature of their morphology and physiology are, perhaps, more likely to be susceptible to short-term acute ozone episodes and these results are discussed in relation to their potential ecological significance.