Aim In this study, we investigated the effects of secondary PET microplastics (< 53 µm) on the Daphnia similis basic life-history parameters survival, age at first reproduction and total offspring number. We also analyzed D. similis enzymatic activity (superoxide dismutase, catalase and glutathione-S-transferase) at sub-effect concentrations. Methods We performed acute and chronic toxicity tests using six PET microplastics concentrations (0, 102, 103, 104, 105 and 106 part. mL-1). We also applied an exposure test to analyze superoxide dismutase, catalase and glutathione-S-transferase activities at sub-effect concentrations. Results D. similis mortality increased (LC50 = 1 x 105 part. mL-1), reproduction decreased (EC50 = 105 part. mL-1) and time to first offspring was delayed by 5 days at the highest microplastic concentration after 21 days exposure. Neither mortality nor alterations in superoxide dismutase, catalase or glutathione-S-transferase activities were detected after 48 h exposure. Microplastics adhered to D. similis body appendages, causing altered swimming behavior. Conclusions Lack of acute toxicity but occurrence of chronic effects serve as a warning for caution when concluding about microplastics non-toxicity in short-term tests. Microplastics had unexpected effects with important ecological implications. Our results contribute to fill the knowledge gaps on the effects of microplastic pollution on aquatic ecosystems, especially under long-term exposure.