CO2 gas exchange, radial growth, chlorophyll (Chl) content and photobiont density of an epiphytic population of Parmelia sulcata were monitored every 2 months during 1 year in a temperate deciduous forest of Central Italy, to verify possible seasonal variations. Light response curves of south-exposed thalli, built up in the laboratory at 6 and 27°C at optimal thallus hydration, showed that CO2 gas exchange changed significantly during the year, with a maximum for gross photosynthesis in December at both temperatures. Photoinhibition phenomena occurred in early spring, immediately before tree leaves sprouted. The principal component analysis of CO2 gas exchange parameters clearly separated the months with from the months without tree canopy cover. Radial growth, measured on marginal lobes of north- and south-exposed thalli, was the highest in December, and the lowest in April. Photobiont density, measured in lobes of south- and north-exposed thalli with a sedimentation chamber, also changed during the year: the number of photobionts was highest in June and December, and lowest in April, although no significant change in cell size and Chl content per cell was evident throughout the year. South-exposed thalli had slightly, but constantly higher photobiont density both on a weight and an area basis. The acclimation of lichen photosynthesis and Chl content to seasonal temperature and light changes should partially be re-visited on the basis of the significant variation in photobiont population density. This phenomenon still awaits, however, a satisfactory explanation, although it is probably related to the seasonal change in nutrient availability.