The intrathalline heterogeneity of chlorophyll a fluorescence (C_aF) in thalli of the foliose lichen Flavoparmelia caperata was studied measuring F_o, F_m, F_v/F_m, ΔF/F_m′, NPQ and F_t in two spots of 25 mm² positioned at ca 0.8 and 3 cm from the thallus margin. Measurements were carried out both in the field and in the laboratory using north-exposed, epiphytic thalli occurring in a deciduous wood of the Karst plateau (Trieste, NE Italy). With the exception of NPQ, all parameters were statistically higher in the marginal spots than in the central ones, suggesting that the level of photochemical activity is higher where the photobiont cells are more dense and younger. F_m and F_v/F_m of marginal spots measured in the field were statistically lower than those measured in the laboratory, probably as a consequence of the different pre-treatment conditions. The importance of selecting a specific measuring spot, particularly for biomonitoring purposes, is discussed.