Increasing studies focus on nanotoxicity, but many unknowns are remaining to be investigated. This study examined toxicities of four varieties of oxide nanoparticles (Al2O3, SiO2, ZnO, and TiO2) to the green algae Chlorella sp. Nanoparticulate Al2O3, SiO2, and TiO2 (DJ3, rutile) had no significant toxicity, whereas nano-ZnO and nano-TiO2 (HR3, anatase) greatly inhibited the algal growth with 6d EC30 of ca. 20 and 30 mg L-1, respectively. Comparative experiments showed the algal toxicities decreased with an order of Zn2+ > nano-ZnO > bulk-ZnO when the concentration of the test material was lower than 50 mg L-1. However, nano-ZnO had higher algal toxicity than Zn2+ ions at concentrations >50 mg L-1 and released less Zn2+ ions into the culture media than bulk-ZnO, which suggests that dissolved Zn2+ ions from nano-ZnO were not the dominant mechanism for the algal growth inhibition. Shading effects of nano-ZnO and HR3 were also excluded from the main mechanism of the nanotoxicity by the comparison of the nanotoxicities in the presence and absence of illumination. Large aggregates of nano-ZnO and HR3 entrapping and wrapping the algal cells were observed, which may contribute to the nanotoxicity. These results will be helpful for understanding the toxicity of oxide nanoparticles. (C) 2010 Elsevier B.V. All rights reserved.