[en] The effect of nanoparticles with the particle size Δ_5_0=5 nm on the test function of aquatic organisms was analyzed by means of biotesting methods with the use of a complex of test-organisms representing general trophic levels. The dependence of an infusoria Paramecium caudatum chemoattractant-elicited response, unicellular algae Chlorella vulgaris Beijer growth rate, Daphnia magna Straus mortality and trophic activity and Danio rerio fish kill due to nNi disperse system concentration, is estimated. It is determined that the release of chlorella into cultivated environment including nNi as a feed for daphnias raises the death rate of entomostracans. The minimal concentration, whereby an organism response to the effect of nNi is registered, depends on the type of test organism and the analysed test function. L(E)C_2_0 is determined for all the organisms used in bioassays. L(E)C_5_0 is estimated for Paramecium caudatum (L(E)C_5_0 = 0.0049 mg/l), for Chlorella vulgaris Beijer (L(E)C_5_0 = 0.529 mg/l), for Daphnia m. S (L(E)C_5_0 > 100 mg/l) and for fish Danio rerio (L(E)C_5_0 > 100 mg/l). According to the Globally Harmonized System hazard substance evaluation criteria and Commission Directive 93/67/EEC, nNi belongs to the “acute toxicity 1” category of toxic substances. (paper)