The effect of the organophosphate insecticide, diazinon on the intramembranous particles (IMPs) of the microvilli of the intestinal epithelial cells of Tilapia nilotica fish was studied using freeze-fracture technique. Exposing fish to different repeated concentrations of diazinon (1/2LC50) caused a significant decrease in population density of IMPs in P- and E-faces. IMPs of microvilli found in intestinal epithelial cells are thought to represent many kinds of proteins including enzymes. In the present work, it is suggested that diazinon induced a reduction in enzymatic content of the membrane which was accompanied by a decrease in IMPs density of the microvilli.