Abstract: The ultrastructural changes of the germinal membrane and the outer surface of experimental Echinococcus granulosus cysts treated with mebendazole for 300 days were observed. Cysts from mebendazole-treated mice were shown by transmission electron microscopy to have marked increase in vacuolization of the distal cytoplasm, dissolution of the minute spines, disturbance of organelles in their arrangement, disrupted aggregates of microtubules, increase in the size and number of lysosomes, decrease in the number of normal appearing Golgi bodies, increased density of the mitochondria, degranulation of the rough endo-plasmic reticulum. Crater-shaped pits, reduced or disappeared microtriches in the surface of the cysts were observed under scanning electron microscope. It seems clear that mebendazole has a profound effect on experimental Echinococcus granulosus cysts in mice. The changes in the structure and the function caused by mebendazole were discussed.