As an attempt to determine if the morphological differences between the southern and western oysters are due simply to the local ecological factors or are based on their fundamental genetic nature, oyster seeds produced in 1968 at Tong-Young, Ye-Chun, and Ko-Hung on the south coast and at Kan-Wol-Do on the west coast were cross-transplanted during May of 1969 to compare their growth. The spats were placed in plastic baskets which permitted free water flow through and the baskets hung from a wooden rack located at a tidal zone of less than I hour exposure at a depth chosen to keep the baskets submerged in water at all times. Twice a month the growth of the spats were measured along with the air and water temperature and salinity. The early summer spats, which were $17-240\%$ larger, in size, than the late summer spats at the time of cross-transplantation, grew more slowly than the late summer spats when exposed to identical environmental conditions, shortening the initial gap to a $5-20\%$ level as the first year of the growth phase came to an end in December. The growth of the Kan-Wol-Do spats lagged considerably behind the southern spats at all localities tested, whereas there were no significant differences among the latter groups. This suggests that the morphological differences between southern and western Pacific oysters in Korea are a manifestation of genetic variety and that Pacific oysters cultured along the south coast are of an identical variety as they are commonly believed to be. The seasonal changes in temperature and salinity even during rainy season in both the southern and western coastal areas are well within the range suitable for successful spawning, and spat fall. However, since the results were based on twice-a-month measurements with no data covering the critical period before and after spawning, they can only serve to indicate at best the general pattern of changes in the environmental conditions of each growing area.