Abstract: Three geographical populations (Pp: Putain population; Dp: Dalian population; Tp: Tokyo population) of Manila clam Ruditapes philippinarum of 3-years old were used to test mass selection on growth in July 2006. Shell length of individual was measured by microscope and vernier caliper, moreover, responses to selection and realized heritability were calculated by the methods of Zheng, et al. (2004, 2006) among three geographical populations of Manila clam. The results showed that the shell length of selection groups were all significantly higher than that of the control group (P＜0.05). R and h_R² of Manila clam decreased with increasing days of age during different development stages, that is SR: Larval rearing stage 0.804±0.084＞Juvenile nursing stage 0.705±0.039＞Youth culture stage 0.671±0.024；h_R²: Larval rearing stage 0.458±0.051＞Juvenile nursing stage 0.402±0.025＞Youth culture stage 0.382±0.013. R of Manila clam was 0.726±0.1074, and Pp, Dp, Tp of that was 0.758±0.101, 0.690±0.049, 0.732±0.059, respectively. Accordingly, h_R² of Manila clam was 0.414±0.044, and Pp, Dp, Tp of that was 0.432±0.058, 0.393±0.028, 0.417±0.033, respectively. The order of R and h_R² of Manila clam among geographical populations was as follow: Pp＞Dp＞Tp. Differences in response to selection and realized heritability among the three geographical populations are presumably due to differences in genetic variability.