High rate of reactive oxygen species (ROS) formation may produce cell damages but the organisms evolved protective antioxidant systems. Among those, superoxide dismutase scavenge superoxide anions. With the aim to study this enzyme in the ciliated protozoan Tetrahymena thermophila, we characterized the genes codifying for two copper-zinc superoxide dismutase (Cu,Zn-SOD). Deduced amino acid sequences were compared to orthologous genes in other species to verify the presence of conserved residues, essential for enzyme activity. Copper-dependent regulation of Cu,ZnSOD expression was investigated by measuring mRNA accumulation and enzyme activity in response to chronic exposure to subtoxic doses of the metal (500 mM). Total RNA and cell-free extract have been purified from T. thermophila cells (SB210 strain) cultured in PPYG medium and harvested after different times from 30 min up to 48 h. Time-course of mRNA accumulations were analyzed by RT-qPCR. The presence of active antioxidant enzyme was verified by measuring Cu,ZnSOD activity spectrophotometrically. The data were correlated to Cu accumulation, measured by atomic absorption spectrophotometry, and in vivo ROS formation, determined after exposure of cells to dihydroethidium. The results emphasise the importance of an efficient detoxification pathway by complex Cu regulation of Cu,ZnSOD transcription, which enables Tetrahymena to survive in the continued elevated presence of Cu in the environment. (Grant by M.I.U.R.)