Sublethal toxicity effects of diazinon on the structure of some organs of great sturgeon ( Huso huso ) were investigated. Diazinon was applied at a sublethal concentration of 1.5 mg L -1 at 22±1 ºC for nine weeks as a constant bath. Tissues of liver, kidney and gills were sampled and examined histopathologically   after 1, 14, 28, 42 and 63 days post exposure to the toxicant. The results of light microscopic examinations of tissues revealed congestion of blood vessels, pyknosis and cloudy swelling in hepatocytes. Congestion of blood vessels and thickening of the basal membrane of bowman capsule in   glumerol of the kidney were also observed. The gill of fish exposed to toxicant also showed congestion   and dilation of blood vessels, swelling of the basement membrane, hyperplasia, fusion and necrosis of   lamellae. This data showed that a long term exposure of fish to diazinon can cause several pathological   changes in respiratory organ, haematopoietic tissues and liver.