The nitrogen fixation (ARA) of specimens of Lobaria pulmonaria growing on Quercus pyrenaica trunks in a mixed wood of beech and oak trees in Montejo de la Sierra (Madrid) is determined. The ARA was analysed monthly for a whole year and incubations were made in situ. L. pulmonaria ARA fluctuates throughout the year and shows two maxima in July and October and two minima in May-June and in August-September . The rate of acetylene reduction and the lichen covering indicate estimable nitrogen inputs.