The increasing use of nanotechnology requires the clarification of the behavior and the effects of nanoparticles (NPs) as they are released into the environment. This study was to investigate the phytotoxicity of cobalt and zinc oxide NPs using the roots of Allium cepa (onion bulbs) as an indicator organism. The effects of cobalt and zinc oxide NPs on the root elongation, root morphology, and cell morphology of a plant, as well as their adsorption potential, were determined through the hydroponic culturing of A. cepa. A. cepa roots were treated with dispersions of the cobalt and zinc oxide NPs having three different concentrations (5, 10, and 20 mu g ml(-1)). With increasing concentrations of the NPs. the elongation of the roots was severely inhibited by both the cobalt and the zinc oxide NPs as compared to that in the control plant (untreated A. cepa roots). Massive adsorption of cobalt oxide NPs into the root system was responsible for the phytotoxicity. Zinc oxide NPs caused damage because of their severe accumulation in both the cellular and the chromosomal modules, thus signifying their highly hazardous phytotoxic nature. (C) 2010 Elsevier B.V. All rights reserved.