Some mixtures of photosynthetic inhibitor herbicides have synergistic effects on the inhibition of the photosynthetic electron transport. Light-sensitive photoprotective ability is involved in the tolerance to single pesticides. Phytoplankton are likely to be simultaneously stressed by light intensity fluctuations and pesticide mixtures in aquatic ecosystems. However, the effect of light intensity on the toxicity of mixed pesticides is very limited. We assessed the influence of light adaptation (40-VLL, 100-LL, and 400-ML µmol photons m-2 s-1) on the toxicity of atrazine and simazine, singly and in combination for three freshwater phytoplankton ( Chlorella vulgaris -CPCC90, Microcystis aeruginosa-toxic-CPCC299 and non-toxic-CPCC632). Toxicity of atrazine and simazine on photosynthesis was greater for the three species grown under the LL condition than under the VLL condition. However, ML-adapted C. vulgaris and non-toxic M. aeruginosa were less sensitive to atrazine and simazine compared to VLL and LL conditions. A mixture of atrazine and simazine produced synergistic (for C. vulgaris), additive (for toxic M. aeruginosa), and antagonistic (for non-toxic M. aeruginosa) effects on the photosynthetic toxicity of the three species at 40 µmol photons m-2 s-1. The interaction in both M. aeruginosa strains shifted to synergism when light intensity increased (VLL-LL, VLL-ML). Under LL and ML conditions, photoprotective ability (NPQ) was extremely sensitive to the inhibitory effects of atrazine and simazine alone, as well as their mixtures. Our results demonstrate that environmental factors (non-chemical) in freshwater habitats can enhance the toxicity of mixed herbicides with the same mode of action on photosynthesis, indicating that light intensity cannot be ignored when assessing the toxic impact of single and binary herbicides on phytoplankton in aquatic ecosystems.