Nano zinc oxide (nZnO) is increasingly used in sunscreen products, with high potential of being released directly into marine environments. This study primarily aimed to characterize the aggregate size and solubility of nZnO and bulk ZnO, and to assess their toxicities towards five selected marine organisms. Chemical characterization showed that nZnO formed larger aggregates in seawater than ZnO, while nZnO had a higher solubility in seawater (3.7 mg L(-1)) than that of ZnO (1.6 mg L(-1)). Acute tests were conducted using the marine diatoms Skeletonema costatum and Thalassiosia pseudonana, the crustaceans Tigriopus japonicus and Elasmopus rapax, and the medaka fish Oryzias melastigma. In general, nZnO was more toxic towards algae than ZnO, but relatively less toxic towards crustaceans and fish. The toxicity of nZnO could be mainly attributed to dissolved Zn(2+) ions. Furthermore, molecular biomarkers including superoxide dismutase (SOD), metallothionein (MT) and heat shock protein 70 (HSP70) were employed to assess the sublethal toxicities of the test chemicals to O. melastigma. Although SOD and MT expressions were not significantly increased in nZnO-treated medaka compared to the controls, exposure to ZnO caused a significant up-regulation of SOD and MT. HSP70 was increased two to fourfold in all treatments indicating that there were probably other forms of stress in additional to oxidative stress such as cellular injury.