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Estimates of genetic parameters in bivalves have typically been based on few families and are limited to larval and juvenile characters. Using mussels from a natural population, 30 replicated families were reared to an average size of 8 mm in the laboratory and subsequently in two natural environments. The effects of rearing full-sibs apart from fertilization and from day 55 have been estimated for adult shell length. Heritabilities were calculated from the sire and dam variance components for mussels up to an average size of 30 mm. In general, the heritabilities for directional were moderate to large, suggesting the possibilities of rapid genetic changes under directional selection. For survival, the additive genetic variance was zero or non-significant. However, the dam component was very large which suggests the presence of either important maternal or non-additive gene effects.