The effect of Paraquat, a herbicide known to stimulate oxygen-centred free-radical production in green plants, is compared with visible injury symptoms due to air pollution in Parmotrema reticulatum, an SO2-sensitive lichen species. This is in order to verify the hypothesized SO2 toxicity mechanism via the generation of free radicals. A new SEM-visible symptom is described: the production of heavy crystalline deposition on the thallus surface in SO2-stressed and Paraquat-treated specimens. The examination of the crystalline deposition by X-ray diffraction, correlated with SEM studies, showed that it is calcium oxalate (weddellite).