The purpose of this study was to determine acute toxicity of cobalt ions (as CoCl2.6H2O) and cobalt oxide nanoparticles (as Co2O3 NPs) as well as their effects on histopathology of gills of zebrafish (Danio rerio) with average weight of 2–3 g. The results of this study showed that the 96-houre LC50 values for both CoCl2.6H2O and Co2O3 NPs were > 100 mg L-1. On the other hand, the results of histopathological studies indicated that exposure of zebrafish to cobalt nanoparticles and cobalt ions caused several gill injuries such as aneurism, dilated and clubbed tips, hyperplasia, oedema, curvature, fusion of lamellae, increase of mucous secretion, hypertrophy, and necrosis. In conclusion, the results of this study showed that the zebrafish is sensitive to both cobalt oxide nanoparticles and cobalt (II) chloride hexahydrate.
