The effect of ultraviolet (UV) radiation on ultrastructure, variable fluorescence of photosystem II and concentrations of photosynthetic pigments in the endemic Antarctic Rhodophyta Palmaria decipiens was studied. After 6 and 8 h of artificial UV-exposure a vesiculation of the chloroplast thylakoids and changes in the membrane structure of mitochondria were observed, which disappeared after 12 h of UV treatment. 60% reduction of maximum quantum yield of photosynthesis (Fv/Fm) was obtained after 4 h of UV-A+B exposure, which recovered up to 80% after 8 h in darkness and 10 h of white light. Exclusion of UV-B led to a minor reduction of Fv/Fm to 60% in the first 4 h of irradiation. The concentrations of chlorophyll a increased after 4 h of UV exposure, whereas concentrations of phycoerythrin and phycocyanin decreased at the same time. After 12 - 16 h of UV exposure no significant changes compared to the control were detectable. These findings give insight into the fine structural and physiological changes during and after UV exposure and indicate an acclimation of the alga to UV radiation.