A breeding-selection program for the clam,Meretrix meretrix,was conducted since 2004.Two of the selection populations were generated with the shell color pattern as an additional selection criterion and named as SP(purple stripes)population and SB(black dots)population.The third-generation SP and SB populations(08G3SP and 08G3SB,respectively)were cultured at two commercial clam farms and a nursery pond and their shell lengths were compared.08G3SB clams had significantly larger sizes than 08G3SP clams at commercial clam farms(p<0.05),yet 08G3SB individuals were significantly smaller than 08G3SP individuals at the nursery pond(p<0.05).Then,we examined the growth of the fourth-generation SP and SB populations(10G4SP and 10G4SB,respectively)at a commercial farm,and found that the shell lengths of the 10G4SB clams increased at a significantly higher growth rate than the 10G4SP clams(p<0.05)from May to September,when the water temperature was between 24.2–27.5 C,while 10G4SB lost the growth advantage in the other months.These results indicated that SP and SB populations responded differently to environmental factors,so it is beneficial for the clam farmers to select a suitable population according to the culture environment.Furthermore,a diallel mating of the SB and SP populations was designed to investigate whether their hybrid population would show heterosis.However,the heterosis was not shown in this study,which might result from the slight genetic divergence between SB and SP populations. A breeding-selection program for the clam, Meretrix meretrix, was conducted since 2004. Two of the selection populations were generated with the shell color pattern as an additional selection criterion and named as SP (purple stripes) population and SB (black dots) population The third-generation SP and SB populations (08G3SP and 08G3SB, respectively) were cultured at two commercial clam farms and a nursery pond and their shell lengths were compared.08G3SB clams had significantly significantly larger sizes than 08G3SP clams at commercial clam farms (p < 0.05), yet 08G3SB individuals were significantly smaller than 08G3SP individuals at the nursery pond (p <0.05) .Then, we examined the growth of the fourth-generation SP and SB populations (10G4SP and 10G4SB, respectively) at a commercial farm, and found that the shell lengths of the 10G4SB clams increased at a significantly higher growth rate than the 10G4SP clams (p <0.05) from May to September, when the water temperature was between 24.2-27.5 C, while 10G4SB lost the growt h advantage in the other months. sese results indicated that sp and SBs are biased only to environmental factors, so it is beneficial for the clam farmers to select a suitable population according to the culture environment. frthermore, a diallel mating of the SB and SP populations was designed to investigate whether their hybrid population would show heterosis. However, the heterosis was not shown in this study, which might result from the slight genetic divergence between SB and SP populations.