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The effects of UV-A on the toxicity of TiO₂ nano-particles (NPs) were evaluated using Moina macrocopa and Daphnia magna under environmentally relevant level of UV-A. The waterfleas were exposed to TiO₂ NPs with different sizes of ~298 nm, ~132 nm, or ~72 nm for up to 48 h, with or without UV-A light. Whole body reactive oxygen species and transcription of antioxidant enzyme genes were measured, as well as the survival of the waterflea. In the presence of UV-A, the survival rates of M. macrocopa significantly decreased in concentration dependent way until ~1 mg/L TiO₂ NPs, but the survivals were reversed at greater concentrations. This peculiar non-monotonic trend of concentration–response relationship might be explained by changes of particle size under different light conditions. TiO₂ NPs within a certain size range could be trapped in the filter apparatus and exert toxicity, and the NPs of greater size …