In Caenorhabditis elegans, although acute toxicity of TiO2 nanoparticles (TiO2-NPs) at high concentrations has been investigated, we still know little about chronic toxicity of TiO2-NPs. Our data here showed that acute TiO2-NPs exposure in the range of mu g/L had no obviously adverse effects on nematodes, but the chronic toxicities of large sizes (60 nm and 90 nm) of TiO2-NPs in the range of mu g/L were detected in nematodes in a modified chronic toxicity assay system. Moreover, chronic toxicities of small sizes (4 nm and 10 nm) of TiO2-NPs in the range of ng/L were observed in nematodes with locomotion behavior and ROS production as endpoints. In nematodes chronically exposed to small sizes of TiO2-NPs at predicted environmental relevant concentrations, locomotion behavior was significantly (P < 0.01) correlated with ROS production. Furthermore, treatment with antioxidants (ascorbate and N-acetyl-L-cysteine) inhibited both the induction of ROS production and the decrease of locomotion behaviors observed in nematodes chronically exposed to small sizes of TiO2-NPs at predicted environmental relevant concentrations. Therefore, chronic exposure to small sizes of TiO2-NPs at predicted environmental relevant concentrations can cause adverse effects on nematodes, and formation of such adverse effects may be largely due to the induction of oxidative stress. (C) 2012 Elsevier B.V. All rights reserved.