This study was conducted on oak trees in 28 localities in Bursa Province. The lichen diversity values (LDV) were determined by the frequencies of all lichen species within a sampling grid placed on tree trunks. Four zones of air quality were determined according to the LDV values. The LDV was decreased in the environments of settlements, roads and agricultural areas. Rural areas were characterised by the Parmelion species, while urban areas were characterised by the Xanthorion and Lecanorion species.