The ultrastructure of hemocytes of Haliotis discus was studied. Five types of hemocytes were recognized: large granule cell, small granule cell, special granule cell, transparent cell and lymphoid cell. According to the morphology of the nucleus and the characters of cell organelle in the cytoplasm, large granule cell and small granule cell are considered the same type cell in different developmental stages, however, special granule cell is early haemocytes of them. Transparent cell and lymphoid cell are completely differentiated cells.