The Pacific oyster Crassostrea gigas is a major aquaculture species in China and around the world.A serious problem in Pacific oyster farming is the occurrence of summer mortalities.Heavy summer mortalities have been observed in cultured Pacific oysters in US,China and France.While the causes for the summer mortality are not well understood.The production and use of triploid oysters may help to reduce summer mortalities.The goal of this study is to determine if heterosis exists between US and China populations of the Pacific oyster and if the interstrain hybrid triploids have greatly improved performance compared to diploids.Hybrid crosses of Pacific oysters between diploids from China and diploids as well as tetraploids from America were conducted.Four groups were produced:Chinese diploid ♀×Chinese diploid ♂,American diploid ♀×American diploid ♂,Chinese diploid ♀×American diploid ♂ and Chinese diploid ♀×American tetraploid ♂.Heterosis and triploid advantage of experimental groups were studied for traits such as fertilization and hatching success,survival,larval and juvenile growth.Our results show that heterosis in diploid hybrid progeny varied among traits and developmental stages:1.21% for larval growth,34.47% for larval survival and 20.39% for juvenile growth.The triploid advantage was significant for all traits and stages,except D larval size(5.19%)and larval growth(4.00%).Triploid advantages for larval survival,larval growth and juvenile survival were 19.92%,30.18% and 54.43%,respectively.The highest triploid advantage(202.96%)was observed in wet weight of juveniles at Day 200.Heterosis and the triploid advantage were stronger at juvenile stages than at larvae stages.Our results suggest that the hybrid triploid Pacific oysters between Chinese diploid females and American tetraploid males have greatly improved growth and survival compared to all diploid groups.Production of triploids through interstrain crossing may be the best way for producing triploids for aquaculture.However,at Day 200 when the triploid advantage was 202.96% in growth and 73.6% in survival,the diploid heterosis was only 3.75% and -26.25%,respectively.This finding suggests that the superior performance of the triploids may not be caused by the same heterosis observed in diploids.It is possible that heterosis in triploids is different from heterosis in diploids.We did not have pure-line triploids as controls in this study.Further studies are needed to determine to what degree that heterosis contributes to the superior performance of hybrid triploids.