Micro and nano-sized plastic particles are found almost everywhere, especially the aquatic system and because of their size, they can be ingested as food by many organisms and these plastics which enter the food chain, raise a great concern. In this study, the effects of 50 nm diameter of polystyrene nanoplastics (PNPs) on Brachionus plicatilis and Daphnia magna were investigated. The acute toxicity tests were conducted on Brachionus plicatilis. According to the tests LC50 value was determined as 1.22 mg / mL (0.34-15.13, 95% confidence limits). Three parameters, tail length, tail intensity and tail moment were evaluated to detect genotoxic effect of PNPs on Daphnia magna with single cell gel electrophoresis (Comet). According to the result PNPs were increased three parameters compared to the control. As a result, PNPs have negative effects on both aquatic organisms Brachionus plicatilis and Daphnia magna.