The occurrence of the larvae, the size of the spats daily attached to the collectors, the difference of the spat size with temperature, and the darly growth of a mussel, Mytilus edulis, were investigated at the off Songpo, Chinhae Bay, the southern part of Korea during the period from March 1 to August 30 in 1986. The water timperature and specific gravity ranged from 7.0 $^{circ}C$ to 27.4$^{circ}C$ (mean 17,24$pm$5.9$0^{circ}C$), and from 1.0126 to 1.0126(mean, 1.0242$pm$0.0023), respectively. D-shaped larvae had two peak occurrences in March 8, April 19, Umbo-shaped larvad three peaks in March 8, April 21 and June 17, and full grown larvae two peaks in May 13 and June 23, respectively. Therefore, it is assumed that the mussel had two mass spawning time in early March and mid April and two mass settling time in mid May and alte June. The maximum size of the planktonic mussel larvae ranged from 375-400${mu}{ extrm}{m}$ and most larvae sizing below 300${mu}{ extrm}{m}$ long settled in adquate substrate. Spats just after settlement had the range from 26.5 to 547.3${mu}{ extrm}{m}$ with the minimum of 225.0 ${mu}{ extrm}{m}$ in mean shell length. The spats settled in summer season, when the water temperature raised above $25^{circ}C$, were smaller than those settled in spring season in mean shell length. The early growth of the mussel spat had been assumed considerably fast.