Thymol monotrpenoid compound is naturally found in many plants. It exhibites molluscicial activity against the land snails and slugs. The present study aims to describe histological, histochemical and ultrastructural investigations to display and clarify any deleterious effects probably evoked post- thymol application in the salivary gland of Limax maximus. To achieve the intended goal in this respect, the slugswere divided into three groups; the first group served as control, the second and the third groups received LC₅₀ & LC₉₀ of thymol for 48 hours in food. The histological results revealed that thymol had adverse effects on type's cells of salivary gland including vacuolated cytoplasm and deformed nuclei in different cells. The histochemical results indicated the presence of neutral mucopolysaccharides in the cells of salivary gland. The ultrastructural results revealed rough endoplasmic reticulum had been fragmented into small stacks and patches heterochromatin also appeared. Therefore, the treated slugs with LC₉₀ of thymol can be recommended for controlling the target slugs and further studies are needed to evaluate their efficacy as safe economic molluscicides in the field, instead of using chemical pesticides that could pollute the environment.