In the district of Dairen, the breeding season of Mytilus edulis starts from November when water temperature falls below 180 C, and continues up to the middle of May of the next year when water temperature rises to about 17℃with two peaks at November- December and April-May, higher in the latter.The optimal temperature for its breeding is about 12-16℃. The heaviest attachment occurs at the end of June to the middle of July. The sex of Mytilus edulis is separate in most of the adults but interchangeable, from female to male as well as from male to female. In this experiment, 3 out of 304 individuals have been found to be hermaphroditic which can shed both egg and sperm at spawning. Germ cells develop from the germinal epithelium of the follicles and accumulate in the follicle. When mature, they are discharged into water and fertilization takes place there. Germ cells develop again after the mature ones are discharged. Thus each individual can breed several times every year. At the end of the breeding season, the gonads become atrophic and eggs at different stages of development undergo gradual degeneration, whereas the interfollicular connective tissue increases. The eggs become first pycnotic, then break into pieces and finally they are taken away by the illvading phagocytic cel1s which penetrate into the egg cytoplasm from the surrounding connective tissue. We had reared the spawning adults in 23 ℃ water for 40 days in winter and found the same atrophic change of the gonads described above. This shows that the atrophy of the gonads in summer is essentially due to the effect of high temperature. Eggs are usually laid at the metaphase of the first maturation division. It is spherical in shape with a diameter of about 68μSperm enters the egg at the vegetal pole. Two polar bodies are formed at the animal pole successively after fertilization. Cleavage is of the spiral type. Polar lobe is formed during each of the first three divisions. D-cell is very much larger than the rest of blastomeres in the 4-cell stage. First quartettet of micromeres is formed after the third cleavage with a dextrotropic spindle. Gastrulation takes place 9 and half hours after fertilization by means of epiboly of the micromeres and the invagination of the derivatives of the D-cell. Pear-shaped trochophore is formed 16 hours after fertilization at 16℃. It is slightly phototropic and moves actively by means of stiff apical cilia and strong prototroch. Veliger larva is formed 2 days after fertilization. At first it is D-shaped, having straight line hinge but gradually becomes oval in shape as a result of asymmetrical growth of the shell. The age at which metamorphosis takes place is varied, some metamorphose as ear1y as 28 days after fertilization, whereas others remain as veliger larva even after 60 days.The size of the larva during metamorphosis also is variable, ranging from 270 to 410μ. The rate 01 development of the larva and growth of the adult is greatly influenced by the temperature, and is faster at higher temperature.