Abstract: Ultrastructure of mantle cells in Hyriopsis cumingii Lea was observed with transmission electron microscope. In the endocuticle, there are 4 kinds of cells; columnar cells with cilia, cupulate cells having small secretory vacuoles in apiculus, columnar cells having large secretory vacuoles in cytoplasm, and columnar cells with piles of mitochondria. The electronic density of cytoplasm and the endocyte of each kind of cells are different, but the nuclei of these cells are ellipsoid or bar-shaped. Generally, mitochondria and Golgi bodies are distributed at free-end. On the free-surface of all kinds of cells, microvilli grow densely. In ciliated columnar cells, there are no secretory vacuoles. On the other hand, cells without cilia have secretory vacuoles of different sizes and electronic density at their apiculus. In exocuticle, only two kinds of cells were observed: common columnar epithelioid cells and cells with piles of mitochondria. Moreover, there are more lysosomes in the exocuticle cells.The ultrastructure of mantle epithelioid cells of Hyriopsis cumingii indicates that both endocuticle and exocuticle cells have secretory and absorptive functions. Endocuticle ciliated columnar cells have motive function in addition. Stronger digestion and absorption are shown in exocuticle cells.