Effects of four different nanocrystalline CeO2-based catalysts on crustaceans Daphnia magna and early-life stages of zebrafish Danio rerio were studied. Pure CeO2 and CuO-CeO2 mixed oxides with a nominal 10, 15 and 20 mol.% CuO content were tested. Pure CeO2 provoked no effects, but CuO-CeO2 mixed oxides induced some sublethal effects on fish and affected daphnids' survival. The most pronounced effects were found on fish body growth, which was reduced at 10 mg/L in case of CuCe20 and 50 mg/L in cases of CuCe10 and CuCe15. Daphnids' survival was affected above 80 mg/L of CuCe20, while CuCe10 and CuCe15 did not affect daphnids. None of the materials was highly toxic to daphnids and fish in comparison to some other environmental pollutants. Differences in effects between the materials could not be explained by their specific physicochemical properties. This work indicates that more attention should be placed at potential toxicity of nanostructured materials. such as nanocrystalline mixed-oxides. (C) 2012 Elsevier B.V. All rights reserved.