A molecular clock based on ITS sequence data from the lichen genera Biatora and Phyllopsora is calibrated with the help of paleoclimatic data and evidence of forest history. The clock indicates that diversification within Biatora started as early as in the Late Cretaceous and took place during periods of climatic cooling, when new types of forest evolved and spread in the Northern Hemisphere. Arctic–alpine species of the genus appear to be of considerable age, dating back to the Late Eocene–Oligocene climatic cooling. By using calibrated phylogenies of epiphytic lichens it may become possible to date many paleoenvironmental events, for which little fossil evidence exists. Key Words: internal transcribed spacers; lichens; Biatora; molecular clock; biogeography.