Abstract: Heteropneustes fossilis (Heteropneustidae) is an air breathing freshwater catfish, has an ever increasing demand as a major source of protein and vitamin A. Acid precipitation has detrimental effect on fish population. We have investigated the activity and localization of the enzyme alkaline phosphatase (ALP), by means of histochemical study on experimental wound repair in the fish skin under acid stress. After injury there was a significant decrease in ALP activity in the cells of the migrating epidermis. There was a gradual increase in the activity of ALP in basal cells, epithelial cells of the middle layer and outer layer and club cells after epithelialization (18 h) of the wound during the early stages of healing. By 96h – 10 days, a fairly high ALP activity in the basal cells was observed. With completion of dermal repair, there was a gradual disappearance of ALP activity. The granulation tissue and the differentiating muscle bundles showed a weak reaction for ALP activity. The late appearance and activity of ALP delayed the wound repair process under acid stress condition.