The effect of the phycobiont Trebouxia arboricola on changes in pigmentation of Xan- thoria parietina (L.) Th. Fr. were investigated in respect of the size the phycobiont and photosynthe- sising pigments in various seasons of the year in insolated and shady places. Observations on the in fluence of the substratum on carotenoid composition in the phycobiont in natural conditions were supplemented with the results obtained in the laboratory. Xanthoria parietina, phycobiont, Trebouxia arboricola, chlorophylls, carotenoids