The Pacific oyster Crassostrea gigas,naturally occurring in Japan,China and Korea,is a major global aquaculture species.It is economically important and has had the highest worldwide production in any cultured aquatic animal species since 1993.China produced over 3.5 million tons of cultured oysters,and C.gigas is one of the most important species.In view of the importance of this species,conventional selections have been carried out worldwide in order to increase growth rate,and to improve survival.However,nearly all the broodstock of oyster used remains unselected in China.In order to improve the productivity traits of C.gigas,a selective breeding program for faster growth was initiated in 2006.From 2007,successive generation of mass selection for fast growth in shell height was conducted in three C.gigas stocks from China,Japan and Korea.The progeny of the third generation of the three C.gigas breeding lines(CS3,JS3,and KS3)were compared with the controls in a 420-day farming experiment.Mean shell height and mean total weight of the oysters from all three selection lines were significantly greater than those of controls(P<0.05),respectively.At harvest on day 420,the increase in shell height for the three selection lines over the controls were 13.4%,10.1% and 10.5%,the increase in weight were 18.5%,13.4% and 11.6%.In the third generation of three C.gigas breeding lines,the realized heritability of shell height were 0.447±0.226,0.471±0.297 and 0.367±0.167.In addition,the genetic gain of total weight for the CS3,JS3,and KS3 was 16.01%±3.82%,15.03% ±5.21% and 11.57%±5.15%,respectively,demonstrating that total weight had been improved markedly during selection for shell height.These results indicate that the selection for fast growth achieved encouraging progress in the third generation of C.gigas breeding lines.