Heterosis between two stocks (A and B) of the bay scallop Argopecten irradians irradians (Lamarck, 1819) was studied by comparing performance in hatching success, larval survival, larval, spat and adult growth of reciprocal hybrid crosses (AB and BA) and that of two parental groups (AA and BB). Stock A is derived from the initial 1982 introduction from Connecticut, USA and expected to be highly inbred because of the small founder population (26 individuals) and the long breeding history (20 generations). Stock B is the fourth generation from a 1999 introduction from Virginia and Massachusetts, USA, which is expected to have higher heterozygosity caused by a larger founder population (200 wild and 206 first generation cultured individuals) and a short breeding history. Both hybrid crosses demonstrated positive heterosis for all traits studied, although the magnitude of heterosis varied among traits and life history stages. Midparent heterosis was 7.9% for hatching success, 10.8% and 3.0% for larval survival and growth and 8.4% and 10.9% for juvenile and adult growth. Single-parent heterosis also varied among traits. For fitness related traits such as hatching success and larval survival, single-parent heterosis was positive for both parental stocks. Single-parent heterosis for growth, a morphological trait, was positive in stock A and negative in stock B.