Scientific research on the toxicity of nanoplastics (NPLs) towards biota has focused mainly ons PS and PE NPLs, and on marine species, remaining a significant knowledge gap on the potential toxicity of other polymers, equally present in environmental samples (e.g., polymethylmethacrylate - PMMA), and on the toxicity of NPLs on freshwater species. Among freshwater invertebrate species, the cnidarians have been poorly studied in ecotoxicology, despite the advantages they offer as model species. Regarding freshwater vertebrates, amphibian species are known to be at rapid decline due to chemical contamination. Therefore, identifying and assessing the extent of potential effects of contaminants (such as NPLs) on these organisms is critical to their preservation. Accordingly, this work aimed at assessing the effects of sublethal levels of PMMA-NPLs on the body morphology and regenerative capacity of Hydra viridissima (cnidarian) and developmental stage and body morphology of Xenopus laevis tadpoles (frog). Morphological changes in H. viridissima were detected at concentrations ≤80 mg of PMMA-NPLs/L. At 80 mg PMMA-NPLs/L, 80% of the hydranths presented severe changes not yet reported in the literature, specifically the presence of doubled tentacles, elbow-like tentacles and curly tentacles. The tadpoles showed a higher sensitivity to PMMA-NPLs comparatively to the cnidarians. Concentrations of 1 mg of PMMA-NPLs provoked severe injuries in the abdomen zone, namely externalization of the gut in more than half of the organisms. The results obtained in this work suggest that cnidarians and early life stages of amphibians are good indicators for assessing freshwater quality in what concerns NPLs. These results provide important baseline information to further understand the toxic potential of PMMA-NPLs to freshwater biota, since it provided results not reported in the scientific literature to date. This work provides important information liable to be framed into more adequate NPLs' risk assessment schemes for the freshwater compartment.