The effect of a sublethal concentration (0.3 mg/liter) of mercuric chloride on the activities of alkaline phosphatase, acid phosphatase, glucose-6-phosphatase, amylase, maltase, lactase, lipase, trypsin, pepsin, aminotripeptidase, glycylglycine dipeptidase, and glycyl- l-leucine dipeptidase in the digestive system of a freshwater catfish, Heteropneustes fossilis, after exposure for 7, 15, and 30 days, and on the percentage composition of total proteins, lipids, glycogen, iron, calcium, inorganic phosphate, water, ash, and vitamins A, D, E, and C in the liver and muscles after 15 and 30 days of exposure was studied. The activities of alkaline phosphatase and glucose-6-phosphatase were significantly decreased while acid phosphatase activity was elevated above the normal level. Significant decreases were also observed in the activities of all the digestive enzymes except for pepsin the activity of which remained unaltered. Maximum inhibition was recorded after 30 days of exposure and the liver was the most affected organ. Total protein and glycogen contents decreased but total lipids increased; calcium, iron, and inorganic phosphate concentrations were below the normal level in fish exposed to mercury. There was significant fall in vitamin A level.