Chlorophyll a fluorescence was measured every three months from December 2003 to December 2004 in ten, randomly selected thalli of the epiphytic lichen Punctelia subrudecta (Nyl.) Krog occurring in a deciduous, thermophilous downy oak wood in southern Tuscany (central Italy). In situ measurements were carried out with a photosynthesis yield analyser Mini-PAM (Walz, Effeltrich, Germany) on the same measuring points of 30-minute dark-adapted, fully hydrated thalli. The results show a remarkable seasonal change in Fo, Fm, and Fv/Fm, probably caused by a long-term physiological-structural adaptation of the photobiont to the gradual modification of the climatic conditions of each measurement period and the light regime of the understorey. However, P. subrudecta is also sensitive to rapid changes in microclimatic conditions as the PSII functionality is affected by the light regime of the days preceding the field measurements. Some critical observations on the statistical analysis of chlorophyll a fluorescence parameters are also made.