An increase in nanomaterial applications will likely lead to an increased probability of environmental exposures, raising concerns regarding the safety of these materials. Recent studies have indicated that manufactured nanomaterials, such as metal oxides, have the potential to be harmful to aquatic and terrestrial organisms. The majority of nano-metal oxide research addressing potential toxicological issues has been focused in aquatic environments with very little terrestrial data. This study characterized the acute and reproductive toxicity of zinc oxide (ZnO) and titanium dioxide (TiO 2) to earthworms (Eisenia fetida) in a terrestrial system. Following a 14 d exposure, nano-sized ZnO on filter paper was acutely toxic to E. fetida, while nano-sized TiO 2 did not exhibit acute toxicity. In contrast, neither nano-sized ZnO nor TiO 2 exhibited acute toxicity to earthworms in sand. Both nano-sized ZnO and TiO 2, following a 4 week exposure, caused reproductive effects in earthworms in artificial soil. Overall, nano-sized ZnO exhibited greater toxicity than nano-sized TiO 2 in Eisenia fetida.