Nanomaterials have significant technological advantages but their release into the environment also carry potential ecotoxicological risks. Carbon-based nanoparticles and particularly diamond nanoparticles have numerous industrial and medical applications. The aim of the present study was to evaluate the toxic effects of diamond nanoparticles with an average particle size of 20 nm on the survival, reproduction and tissue structure of the freshwater crustacean Daphnia magna. The chronic toxicity test results showed 100% mortality at concentrations higher than 12.5 mg l(-1) and that reproduction inhibition occurred in concentrations higher than 1.3 mg l(-1). Light microscopy showed that diamond nanoparticles adhere to the exoskeleton surface and accumulate within the gastrointestinal tract, suggesting that food absorption by the gut cells may be blocked. The results support the use of chronic approaches in environmental protection as part of an integrated environmental monitoring and assessment strategy.