AbstractOffspring from a mass spawning of the Chilean oysterOstrea chilensis(Philippi) were reared in three locations, at two depths and three densities along the east coast of Chiloé Island, southern Chile, to test for the presence of genotype‐environment interaction for growth rate and the effect of environmental variables on shell growth. Site, density and depth effects and their interactions upon shell length variation at 26 months of age were all statistically significant (P<0.05). These results indicate that sibling oysters perform differently in different environments, densities and depths. The shell length ofO. chilensisin the three different environments may be limited by food supply when the abiotic variables are in the range of tolerance for the species. Salinity was the most important factor limiting the shell growth ofO. chilensisin the Quempillén estuary. Thus, in breeding programmes, it is recommended that the testing of stocks and their offsprings’performance should be carried out on the farm to ensure that the selection criteria are