In this paper, histopathological changes in the inner lining of the accessory respiratory organ of Heteropneustes fossilis following exposure to sublethal concentration (0.2 gl sup(-1)) of ammonium sulphate (3 mg l sup(-1) total ammonia-N) has been described. The goblet cells show periodic increased followed by decreased secretory activities. Necrosis and shedding of the epithelial cells over the secondary lamellae cause periodic haemorrhages which lead to degeneration and decreased number of secondary lamellae. Subsequently regeneration takes place each time as evidenced by the appearance of inflammatory tissue. Fusion of more than one secondary lamellae is also common. Regeneration also leads to uncontrolled hyperplasia of haphazardly arranged epithelial cells. This hyperplasia causes increased distance of respiratory blood-air barrier in the secondary lamellae, leading to impaired normal aerial respiration.