Photoinhibitory effects of ultraviolet radiation (UVR) on four Cosmarium strains were studied with respect to their geographical distribution pattern. This study dealt with two strains of a cosmopolitan taxon (C. punctulatum var. subpunctulatum) collected from high-mountain tropical and lowland polar regions, one typical tropical species (C. beatum) and one typical polar representative (C. crenatum var. boldtianum). Physiological characteristics of the strains during and after various UVR spectral combinations at two temperature gradients were determined by the measurement of chlorophyll fluorescence, oxygen evolution rates and using an inhibitor of chloroplast-encoded protein synthesis (streptomycin). All of the Cosmarium strains investigated exhibited consistent geographical distribution patterns in accordance with the UVR prevailing at their sampling sites, despite a long-term cultivation under constant laboratory conditions. It appeared that moderate ultraviolet-B radiation (UVBR) treatment did not exert large damages to photosystem II in all of the Cosmarium strains, compared to ultraviolet-A radiation (UVAR) treatment at 21 °C. Interestingly, an ameliorating effect of UVBR at 21 °C was observed in C. beatum as concluded from higher rates of recovery of maximum quantum yield after moderate UVBR treatment, compared to that after UVAR application. This study also reveals that the mucilage of desmids has a limited role in the protection against UVR as demonstrated by the measurements of absorption in the UVR range, in contrast to previous assumptions. Increased UVBR (i.e. high UVBR : PAR ratio) severely decreases oxygen evolution in all of the Cosmarium strains, pointing to possible consequences for peat bogs which are native habitats of desmids, as they are particularly poor in oxygen.