Lichens are phototrophic organisms tolerant to adverse environmental conditions. However, the mechanisms underlying their stress tolerance are not fully understood. For photosynthetic organisms depending on solar radiation, UV-B radiation (280–320 nm) acts as a stress factor. We studied the pro-/antioxidant and respiratory metabolism of Peltigera aphthosa to identify adaptive responses of lichen to a physiological dose of UV-B radiation (14 kJ day− 1 for 10 days). A browning of the upper cortex, the appearance of dark spots in the medulla layer of treated thalli, and an increase in the browning reflectance index indicated the synthesis of protective UV screening pigments. UV-B treatment did not cause significant changes in the photosynthetic activity of thalli and isolated algal cells. More intense lipid peroxidation activity and transient changes in H2O2 content accompanied the acclimation process. Higher superoxide dismutase and catalase isoenzyme levels and activity were noted 4 days following the termination of the UV-B treatment. Increased alternative respiration capacity (AP) and a contribution of this energy-dissipating respiratory pathway of up to 45% of the total respiration rate were noted in treated thalli, but not in isolated algal cells. These data demonstrate the UV-B effect on the Peltigera aphthosa respiratory metabolism to be higher due to reactions of the mycobiont than those of the photobiont. We suggest that the activation of the energy-dissipating AP in mycobiont mitochondria may be associated with the synthesis of protective pigments. Keywords: UV-B radiation · Oxidative stress · Antioxidant enzymes · Alternative and cytochrome respiration · Photobiont and mycobiont · Peltigera aphthosa.