Background: Silicosis is a fibronodular lung disease caused by inhalation of dust containing crystalline silica (silicon dioxide). Aim of the work: To investigate the changes in the lung alveoli after silica dust inhalation on the ultrastructural level in rats. Materials and Methods: Thirty adult male albino rats were used and divided into 2 groups (15 animals each); the 1st group (Group I) served as a control group, silica dust (SiO2) was administrated by inhalation to the 2nd group (Group II) (10 times daily for 6 weeks). Lung specimens were processed for study by electron microscope. Results: After silica treatment, features of thickening of the interalveolar septum by cellular and connective tissue infiltration were manifested. Ultrastructural changes were observed in the form of cellular abnormalities represented by degeneration, vacuolation, dilatation of the rER and clarification of the cytoplasm of type II pneumocytes. Cellular infiltration by neutrophils, eosinophils, lymphocytes, mast cells and macrophages was also observed. Furthermore fibrosis manifested by collagen deposition has been found. Conclusion: Silica is directly cytotoxic to pulmonary target cells. We should always remember that if it is silica, it is not just dust and avoid exposure.