The measurements of chlorophyll fuorescence play an important role in studies of lichen physiology. Usually, for foliose lichens fuorescence kinetics is recorded from the upper thalline side often exhibiting green color refecting the presence of photosynthetic pigments. The lower side of such lichens is grey, dark-brown or black. At the frst time, we evaluated photosynthetic activity distribution by chlorophyll fuorescence analysis on both lower and upper thallus sides for the foliose lichen Nephroma arcticum. We have demonstrated that photosynthesis proceeds not only on the green-colored upper side, but also on the gray lower side of the curled growing edges of the thallus lobes. These sides were difered in terms of PSII photochemical quantum yield, activity of non-regulatory dissipation and non-photochemical quenching of excited chlorophyll states (NPQ). Upper side was characterized by higher maximal PSII efciency, whereas the lower one of the curled edges was characterized by higher actual photochemical quantum yield during actinic light acclimation. NPQ was higher on the upper surface, whereas, on the lower side (of the curled edges) non-regulatory dissipation was predominant. In terms of photosynthetic activity measurements, these results show, that actinic and measuring light reached the layer of phycobiont despite its shielding by mycobiont hyphae. On the melanized lower side in the basal thalline zone attached to the substratum photosynthesis was not detected. Lower side demonstrated higher level of light scattering in the refectance spectra. We believe that diferent photoprotective mechanisms against high light are crucial on the upper and lower sides: NPQ on the upper surface, and light scattering and shielding by mycobiont on the lower side. Possible biological role of photosynthesis on the lower side is discussed. Keywords: Nephroma arcticum · Lichen photosynthesis · Foliose lichens · Non-photochemical quenching · Chlorophyll fuorescence.