The thalli of 24 lichen species, collected from unpolluted and polluted suburban areas, were studied by light and electron microscopy in order to detect their morphological and physiological alterations due to air pollution. The effects on photosynthesis were determined by measuring the chlorophyll content of the algae using the autofluorescence test and by the ultrastructural observations of the thylakoid system. The element distribution in the thalli was analyzed by semiquantitative X-ray microanalysis applied to a scanning electron microscope. From our results, it seems that air pollution causes a remarkable drop in chlorophyll content as recorded by measuring the algal autofluorescence and appreciable alterations of the photosynthetic apparatus such as the swelling of the thylakoids observed by electron microscopy. Moreover, X-ray microanalysis shows that the elements always detected in significant amounts are: aluminium (A1), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), and calcium (Ca). The element concentration varies in the different portions of the thalli: magnesium (Mg) is located mainly in the algal layers and A1 in the reproductive structures (apothecia) and in the cephalodia, where cyanobacteria are present. The amount of Mg and iron (Fe) is lower in the thalli of Cladonia from unpolluted areas than in those from polluted areas; Cladonia furcata and Dermatocarpon miniatum have a high Si content; Cladonia has also the highest amount of heavy metals and S. Among the techniques used in our research, electron microscopy is the most sensitive one for the documentation of cellular damages; X-ray microanalysis is indispensable to detect heavy metals, but the autofluorescence test seems to be the most useful for a rapid control of the effect of the air pollution on lichen morphology and physiology. Lecanora dispersa, Lecanora hagenii, Physcia adscendens, and Xanthoria parietina are the more tolerant lichens studied.