Lichens are sensitive organisms, widely applied in air quality assessments and monitoring programs around the world. Traffic emissions or articulate matter influence the pattern in presence of lichens on roadside trees. The research of epiphytic lichens in Prešov city (Slovakia) was conducted in 17 near-road localities (three trees per locality). The traffic density, selected tree parameters, lichen density, and presence in research squares (15 x 15 cm) on 4 sides of the tree (side in the direction of traffic, in the opposite direction of traffic, the side facing towards the road and the side facing away from the road) were analysed. Three lichen species were most often determined on the trees: Xanthoria parietina (L.) Th. Fr., Phaeophyscia orbicularis (Necker) Moberg, and Physcia adscendens (Th. Fr.) H. Olivier. The most common lichen was the Ph. orbicularis accounted for up to 86% of all records. The highest presence was confirmed in localities with the highest traffic density. Based on the results it is obvious that atmospheric pollution alters lichen communities and therefore can be effectively used as biological indicator of air quality. Keywords: urban area, Phaeophyscia, Physcia, Xanthoria, traffic density.