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 Zinc oxide (ZnO) nanomaterials (NMs) are widely used in the manufacture of several commercial products like foods, packaging, cosmetics, medicines and healthcare formulations and anti-fouling paints. These NMs can pollute water bodies when they become bioavailable. In this context, this study investigated the toxicity of ZnO nanorods (NRs) on green microalgae from freshwater and marine ecosystems, to better understand the behavior of this NM on each environment. Two green microalgae species, Desmodesmus subspicatus (freshwater) and Tetraselmus sp. (marine), were evaluated by chronic toxicity tests and oxidative stress induction by the enzymatic activity of catalase (CAT). The exposition assays were performed using three different concentrations of ZnO NRs (0.1, 1.0, and 10 mg/L, and a negative control). ZnO NRs significantly affected the growth rate of both tested chlorophytes. The chronic toxicity test showed LOEC (Lowest Observed Effect Concentration) levels of 10 mg/L (72 h) for D. subspicatus and 1.0 mg/L (24 h) for Tetraselmissp. It was observed NOEC (No Observed Effect Concentration) levels of 1.0 mg/L to D. subspicatus was (at 72 h) and of <0.1 mg/L for Tetraselmis sp. (at 24 h). In the enzymatic activity tests of D. subspicatus exposed to ZnO NRs, the CAT activity caused significant changes at the concentration at 10 mg/Lof ZnO NRs when compared to the control test, but for Tetraselmissp. no change was observed in CAT activity. These results indicate that D. subspicatus was more sensitive to the effects of ZnO NRs at the concentration of 10 mg/L after 72 h, while oxidative stress of this alga was also observed at the same concentration. The results of this study show the importance of further investigating the toxicological effects of ZnO NRs on green microalgae from distinct aquatic environments and of evaluating the toxicological response of these microalgae in culture media.