To understand the interaction of titanium dioxide nanoparticles(nTiO₂)and copper(Cu²⁺)and their biological effects, using Daphnia magna as a test animal, we studied the joint toxicity effects of nTiO₂ and Cu²⁺ on Daphnia magna and its internal antioxidant system. The results show that the respective values of 72 h-LC₅₀ and 72 h-EC₅₀ of Daphnia magna exposed to nTiO₂ were 22. 742 and 16. 739 mg/L and the values of Daphnia magna exposed Cu²⁺ were 11. 635 and 10. 129 μg/L. When nTiO₂and Cu²⁺ coexisted, the respective values of 72 h-LC₅₀ and 72 h-EC₅₀ of Daphnia magna exposed to nTiO₂ were 12. 721 and 8. 827 mg/L and the values of Daphnia magna exposed to Cu²⁺ were 12. 405 and 10. 559 μg/L. When Daphnia magna were exposed to both nTiO₂ and Cu²⁺, the activity of superoxide dismutase(SOD)and reduced glutathione(GSH)was first induced and then inhibited, and the malondialdehyde(MDA)content produced during lipid peroxidation increased gradually. This indicated that the antioxidant system had been attacked by reactive oxygen species(ROS), and it could not provide protection for Daphnia magna, which was the key reason for the joint toxicity of Cu²⁺ and nTiO₂.