The increase in the penetration of ultraviolet radiation that has been reported lately for freshwater ecosystems in southern South America would allegedly generate alterations in ecological processes. In this respect the mortality of Parabroteas sarsi, a calanoid copepod distributed in South American lakes and ponds, was studied. Specimens were reared at two different concentrations of dissolved organic carbon and subjected to 72 h exposure to artificial ultraviolet radiation. At high dissolved organic carbon concentration the mortality of P. sarsi increased at 24 h and stabilized at 48 and 72 h, whereas at low dissolved organic carbon concentration mortality increased linearly during the studied period. The results support both the description of a screen effect of dissolved organic carbon against ultraviolet radiation, and the potential photorepairing role of visible radiation that explains the increase of mortality in conditions of low dissolved organic carbon concentration.