To determine the SO2 resistance of lichens pertaining to different groups of growth form their thalli were exposed to SO2 in air (4 mg/m3) (determination of total resistance) or were submerged in Na2S2O5 solution (determination of plasmatic resistance). As a viability criterion the CO2 exchange of the thalli was measured with an infrared gas analyzer before and after SO2 exposure or sulphite treatment. 4 crustose lichens, 5 foliose lichens, 3 fruticose lichens and 1 foliose gelatinous lichen were tested. The resistance of the individual species as determined by the fumigation experiments and the sulphite treatment varies considerably, also within the usually employed growth form groups. An arrangement of the species in order of their plasmatic resistance results in another sequence than an arrangement in order of their total resistance. Of all the tested species the crustose lichen Rhizocarpon geographicum showed by far the highest plasmatic resistance. Although crustose lichens show a tendency to a greater resistance and fruticose lichens to a lower resistance to SO2 gas, there is no high correlation between growth form and SO2 sensitivity. Neither can crustose lichens be considered as generally more resistant than foliose ones nor can foliose lichens be considered as more resistant than fruticose species, because some of the tested lichens did not fit in this often accepted sequence of decreasing resistance. Some crustose lichens (e. g. Pertusaria corallina) were for instance found to be more sensitive than foliose lichens (e. g. Xanthoria parietina), several foliose lichens (Parmelia glabratula, Lobaria pulmonaria) were less resistant than beard-like lichens (Alectoria fuscescens, A. pubescens). After SO2 treatment the gelatinous lichen Collema cristatum is totally damaged. It is possible that anatomical-morphological features of the lichens influence the SO2 resistance, and some results support this, however the species are still too manifoldly differenciated within the roughly defined growth form groups to reveal a high correlation between growth form and SO2 sensitivity. It seems that lichens of moist and shady habitats are more easily damaged than those of dry sites. Epiphytic and silicate lichens apparently do not differ basically in their SO2 resistance. Lichen species with a large ecological amplitude are best qualified for bioindicators of SO2 pollution; lichens with a narrow ecological range are too sensitive to environmental changes not caused by SO2.