Abstract The garden snail ( Helix aspersa ) is currently used as bioindicator of metallic pollution. Our objective was to extend its use to organic chemicals by studying the effects and tissue concentrations of the organophosphorus pesticide dimethoate follo wing dietary uptake. After exposure for four weeks to increasing doses of pesticide in the diet, the median lethal concentration (LC50) was determined to be 3,700 μg/g food. Clinical signs indicated a no‐observed‐effect concentration of 100 μg/g and a lowest‐observed‐effect concentration of 250 μg/g. The growth parameters were decreased with increasing exposure to the pesticide. The median effective concentration (EC50), which was evaluated based on both shell diameter and dry weight inhibitions, was 665 and 424 μg/g, respectively, and the EC10 was 180 and 145 μg/g, respectively. Accumulation in the viscera was related to the amount of dimethoate in the food. The bioconcentration factors were low (>6 × 10 −3 ). Acetylcholinesterase (AChE) activity was strongly decreased (80% from 250 μg/g). In conclusion, we demonstrated that the species H. aspersa could be a useful sentinel organism for organophosphorus contamination surveys. Among the effects measured, the inhibition of AChE activities and clinical signs were the most sensitive, followed by the growth parameters. These results confirm the suitability of the garden snail for development of sublethal toxicity tests using primary consumers and aboveground organisms.