Abstract: The present study was designated to compare the ability of the two carbamate compounds methomyl and carbofuran, the organophosphorus compound chlorpyrifos and the bipyridylium compound paraquat to induce the oxidative stress and affect some biochemical targets in the terrestrial snail, Helix aspersa. LD50 values for these pesticides were determined 48 h following topical application. They were 240, 500, 900 and 920 μg/snail for methomyl, carbofuran, chlorpyrifos and paraquat, respectively. Some biomarkers of the oxidative stress such as Lipid Peroxidation (LP), Lactic Dehydrogenase (LDH) and Glutathione (GSH) as well as the inhibitory effects of these compounds against acetylcholinesterase (AChE) were carried out following topical application of 1/4LD50 values. The results showed that carbofuran was the most potent to inhibit AChE in snails followed by methomyl, where the enzyme activities dropped to 9.86 and 28.82% of the control activity, respectively, 48 h following application. Non-significant increase in the levels of thiobarbituric acid-reactive species in the snail tissue homogenate intoxicated with methomyl, paraquat or carbofuran were found comparing to control value, while it was similar to control following chlorpyrifos treatment. On the other hand, the activities of LDH were increased following all tested pesticides. Also, the results showed that GSH level in the snail tissue homogenate was elevated following both methomyl and chlorpyrifos, while it decreased following either carbofuran or paraquat application. It could be concluded that methomyl was the most toxic pesticide followed by carbofuran against the land snail and their mode of action could be due to the induction of oxidative stress in addition to their anticholinesterase potencies. Chlorpyrifos or paraquat had slightly effects to alter the biomarkers of oxidative stress in the snail.