Three lichen species, Hypogymnia physodes, Bryoria capillaris and Peltigera canina were fumigated with 215 μig m−3 of SO2 (6 h per day, 5 days per week) in exposure chambers under three different moisture conditions. The treatments were (1) low humidity (40–50° r.h. continuously), (2) high humidity (80–90° r.h. continuously) and (3) high humidity + water (80–90° r.h. + daily moistening). The value of light microscopy, fluorescence microscopy and electron microscopy in assessing the SO, injury in the lichens was compared. The three lichen species studied were all damaged by this level of SO, fumigation, and injury increased with increased duration of exposure. Electron microscopy was the most sensitive method of revealing cellular injuries after 5 days of exposure in all the species studied. Fluorescence microscopy revealed changes in the colour and intensity of fluorescence after 10 days of exposure in the two epiphytic species, but no clear changes in the cyanobacterial cells of P. canina could be detected by this method. Light microscopy revealed plasmolysis of the algal cells in the two epiphytic species after 10–15 days of fumigation. All the species studied were clearly more sensitive to SO2 when cultured in air of high humidity, although slight cellular injuries also occurred in the dry air treatment (40–50° r.h.). Additional daily moistening tended to protect against injury in the lichens cultured at high humidity. In the present continuous flow fumigation chamber P. canina proved to be sensitive to SO2 in regard to ultrastructural injuries that could be observed in both the cyanobacterial and fungal cells.