The results of a lichen bioindication survey of air quality performed in the Mt. Amiata geothermal area (Tuscany, central Italy) are reported. On the basis of 153 Index of Atmospheric Purity (IAP) samplings, an air quality zonal map of the area was drawn. The lowest IAP values were recorded in an area encompassing the geothermal power plants, up to a distance of about 500 m. The overall pattern of rising IAP values with increasing distance from the geothermal power plants suggested that air pollution from the geothermal installations is the main cause of the observed zonation of lichen communities. It is suggested that hydrogen sulphide is the main contaminant responsible for lichen decline around geothermal power plants.