Abstract: Abstract: Two inbred populations (UU and MM) and one cross-bred F1 population (UM) were constructed using brood stocks from the US and ME populations. Their growth parameters were measured on Day 112 and 164 and the major factors affecting their body weight were analyzed. The results indicated obvious growth hetereosis in shell height, shell thickness and whole weight (P<0.05), suggesting that it is possible to improve the production traits in bay scallop through inter-population hybridization. Path analyses showed that for inbred populations, shell length was the major factor affecting whole weight. For the cross-bred population, however, shell height was the major factor affecting whole weight on Day 112 while shell thickness was the major factor on Day 164. Therefore, different parameters should be used in selection of the cross-bred population at different stages to ensure the greatest hetereosis.