Biodiversity studies on lichen-forming fungi at Gyűrűfű (SW Hungary). – Biodiversity of lichen-forming fungi was studied in October 2008 in the framework of the 4th Hungarian Biodiversity Day. Lichens from the bark of deciduous trees were sampled in the investigation area of 1 km2, saxicolous species were observed on concrete, and terricolous species were missing. Altogether 53 species were found. Comparing this number to the total number of species of all investigated organisms (2,900), lichens represent 1.8% of the biodiversity of the study site. Due to the low number of protected lichen species (8, most of them terricolous) in Hungary, protected lichen species were not found. The following rare species were identified: Bacidia friesiana as new to Hungary, Lepraria rigidula as the second, Ochrolechia arborea as the third, and Hyperphyscia adglutinata as the fifth record from Hungary. Key words: Bacidia friesiana, biodiversity, Gyűrűfű, Hungary, Lepraria rigidula, lichen-forming fungi, Ochrolechia arborea