Bioindicators have not yet been widely used to measure the improving atmospheric conditions in our cities. This paper describes a study of the reinvasion of one lichen, Lecanora muralis (Schreb.) Rabenh., into the West Yorkshire conurbation. The results indicate that the response to improved atmospheric conditions is slow and difficult to model in terms of a simple threshold value of sulphur dioxide alone. It appears that the lichen's response becomes dependent upon other parameters as the atmospheric pollutant levels drop.