Plastic nanoparticles (NPs) released from plastic pollution pervade aquatic ecosystems, raising concerns about their long-term toxic effects on aquatic organisms. The purpose of this study was to understand the sublethal toxicity of polyethylene nanoparticles (PENPs) and polypropylene nanoparticles (PPNPs) of the same size (a 50 nm diameter) in Hydra vulgaris. Hydras were exposed to increasing concentrations of the PENPs and PPNPs (0.3-10 mg/L) for 96 h at 20 ^oC. Toxicity was determined based on characteristic morphological changes and a gene expression analysis of genes involved in oxidative stress, DNA repair, protein salvaging and autophagy, and neural activity and regeneration. The data revealed that the PPNPs produced morphological changes (50% effect concentration EC50 = 7 mg/L), while the PENPs did not. Exposure to both nanoplastics produced changes in gene expression for all gene targets and at concentrations < 0.3 mg/L in some cases. The PPNPs generally produced stronger effects than the PENPs. The mode of action of these plastic polymers differed based on the intensity of the responses in oxidative stress (superoxide dismutase, catalase), DNA repair of oxidized DNA, regeneration, and circadian rhythms. In conclusion, both types of plastic nanoparticles produced effects at concentrations well below the appearance of morphological changes and at concentrations that can be found in highly contaminated environments. This study also show that hydra are very sensitive to plastic nanomaterials.