The histopathological changes in liver, stomach, intestine, kidney, and gills and the hematological changes produced by two sublethal concentrations of diazinon (0.2 and 0.4 mg/liter) in a freshwater teleost fish, Ophiocephalus punctatus, have been studied after 15 and 30 days of exposure. The most conspicuous pathological changes in the liver after 15 days were liver cord disarray, rupture of cell membrane, vacuolation in the cytoplasm of hepatocytes, and enlargement of nuclei. The liver damage became more severe in fish exposed for 30 days and was characterized by fibrosis, small degenerated nuclei, and invasion of a number of phagocytes into the necrosed tissue. The only visible change in the stomach was erosion of mucosa. In the intestine, a few villi were necrosed and nuclei were reduced in volume. In the kidney, shrinkage of the glomerular network and necrosis of proximal tubules were the most conspicuous changes. The distal and collecting tubules were ruptured and the nuclei were degenerated and pycnotic. Degeneration and thinning of epithelial lining of the lamellae, separation of the vascular layer from the epithelium, and in some cases fusion of the epithelium of adjacent villi were the alterations in the gill. In blood, hemoglobin, hematocrit, glucose, cholesterol, urea, sodium, and calcium levels increased while plasma proteins, chlorides, and inorganic phosphate remained unaltered. Among the serum enzymes, the activities of glutamic-oxalacetic transaminase, glutamic-pyruvic transaminase, alkaline phosphatase, and amylase increased while acid phosphatase and cholinesterase activities decreased on exposure to diazinon.