1. Ecological studies of the seed Clams of Meretrix lusoria and Cyclina sinensis were undertaken at Songdo, Inchon, from December, 1967 to April, 1971. 2. In Cyclina sinensis and Meretrik lusoria about 10 percent of the eggs are mature in the be ginning of August ; however, in the middle of August the number of mature eggs increased to 20 percent in the former species and 15 percent in the latter. 3. The spawning rate was not so high in the beginning of August. However the rate rose to almost 10) percent after the middle of August, when a NH₄OH sea-water solution of 1/10∼3/100 Normality was injected into the gonads of clams of the two species mentioned previously. 4. The author confirmed that the artificial fertilization rate resultant from the NH₄OH sea-water treatment of Cyclina sinensis was high after the middle of August ; however, in the case of Meretrix lusoria the fegtilization rate was bigh after the beginning of August. 5. The appearance frequency of the planktonic larvae of the two species was high during the period from the middle of August to the beginning of september. 6. The maximum population size of the young clams -1∼25mm in shell length- of Cyclina sinensis was 396 per lm² in July, 1970 in Namdong Salt Farm Reservoir ; however, in the case of Meretrix lusoria, the maximum population of young clams -less than 2mm in shell length was found to be 4700 per lm², on th west side of Aam Island, Songdo, Inchon, in December, 1970. 7. The author confirmed that the population size of the young clams -03∼25mm in shell length- was greater in December than in November ; however, the population size decreased sharply in April after hibernation. On the other hand, the number of dead young clams of the species increased markedly after hibernation. 8. The author noted that the settled young clams of Cyclina sinensis reached 12∼13mm in shell length after 1 year and in the case of Meretrix lusoria, 13∼l6mm, two years was found to be necessary for the clams of both species to attain the size of 20mm in shell length before they could be used as seed clams.