Biological effects and ecotoxicity of NPs CeO2 with particle size Δ50 =16 nm and hydrobionts tolerance was measured in simulation based on test-reactions of a group of organisms representing major trophic levels of aquatic ecosystem. Biological activity of NPs CeO2 was noted against bacterial biosensor Ekolum, Сhlorella v. B.,Parameciumc., Daphniam. S. and Danior. Toxicity and hazard level of NPs CeO2 were measured using biotesting and hydrobionts. Toxicity of NPs CeO2 against luminescent bacteria Ekolum and Parameciumc. and Danior. was not established (L(E)С50 > 100 mg/l). In accordance with SGS and EC 93/67/EEC, NPs CeO2 is considered highly toxic (acute toxicity Level 1). Biomarkers and NPs CeO2 tolerant hydrobionts were determined. Range of tolerance of hydrobionts to NPs was established. The upper limit of tolerance was determined for certain hydrobionts by values of L(E)С50: 0.009 mg/l (by volume of Chl a) for Сhlorella v. B. and 46.15 mg/l (by mobility) for Daphniam. S.