In the present study, the acute and developmental toxicities of imidazolium ionic liquids (ILs) with different alkyl chain lengths, as well as the antioxidant response and lipid peroxidation levels were evaluated in the snail, Physa acuta. Longer alkyl chains corresponded to increased IL toxicity in snails. Long-term IL exposure at lower concentrations inhibited snail growth and reproduction. We also found that IL inhibited the activities of superoxide dismutase (SOD) and glutathione S-transferase (GST), promoted the activity of catalase (CAT), and increased the glutathione content. However, SOD, GST, and CAT activities returned to control levels after 96 h of recovery. In addition, malondialdehyde levels were increased in treatment groups compared with the control and did not return to control levels even after a recovery period, indicating that ILs induced lipid peroxidation in snail viscera. These results suggest that oxidative stress and lipid peroxidation may be involved in the mechanism of toxicity for ILs.