Abstract Background: Chemicals have deleterious effect on the environment. The wide use of nanomaterials as products for plant protection, fertilizers, and also in water purification leads to the release of these materials to the environment. Terrestrial gastropods including snails and slugs have the ability to accumulate heavy metals in their bodies. The present study evaluates the toxic effect of zinc oxide nanoparticles on the terrestrial slug Lehmannia nyctelia. Zinc oxide nanoparticles (ZnO NPs) were prepared by thermal decomposition method. ZnO NPs are characterized by Xray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and ultraviolet-visible spectroscopy (UV). Slugs were treated with three different concentrations of ZnO NPs. Results: A total of three animals died by the end of the experiment. Many histological alterations were detected after exposure to different concentration of ZnO NPs. Conclusions: The obtained histological alterations prove the toxic effect of ZnO NPs on the animal under study