Chromium III oxide (Cr2O3) nanoparticles (NPs) are used in pigments for ceramics, dyes, paints and cosmetics. However, few studies addressing the toxic potential of these NPs have been reported in the literature. Thus, this research aimed to evaluate the acute and chronic effects of Cr2O3 NPs through acute toxicity tests with Daphnia magna and Aliivibrio fischeri and chronic toxicity tests with Daphnia magna. Cr2O3 NPs were synthesized by the sol–gel method and characterized through TEM, X-Ray diffraction (XRD), zeta potential (ZP) and surface area analysis. In the acute toxicity tests the EC50,48h value obtained with D. magna was 6.79 mg L−1 and for A. fischeri the EC50,15min value was 16.10 mg L−1 and the EC50,30min value was 12.91 mg L−1. Regarding the chronic toxicity tests with D. magna, effects on longevity (OEC=1.00 mg L−1), reproduction (OEC=1.00 mg L−1) and growth (OEC=0.50 mg L−1) were observed. On the SEM and TEM images, ultrastructural alterations in the organelles of exposed organisms were also observed. Thus, toxicological studies with NPs are of great importance in order to reduce the risk of environmental contamination.