Genotype x environment (G x E) interactions affecting weight in Pacific oysters grown in five environments, comprising two farms in South Australia (SA₁ and SA₂), and three farms in Tasmania (Tas₁, Tas₂ and Tas₃), were estimated. Body weights were measured at five time points between approximately 1 and 2 years of age (T₀, T₁, T₂, T₃, and T₄) in a four generation population, from a breeding program based mainly on full sibling matings. Families were grown in the same rearing environment until T₀ when they were selected at random into family batches of size 100 and distributed to the five farms. Data analysed were average animal weight derived from total batch weight. G x E interactions were present as scale effects, with greater variation between families in high growth environments in South Australia relative to Tasmania. These effects increased with time, such that by T₄ the estimated family variance on the South Australian farms was approximately five to seven times greater than on Tasmanian farms. Apart from one environment at T₄, the estimated genetic correlations between environments averaged 0.91, 0.91, 0.90 and 0.91 at times T₁, T₂, T₃ and T₄ respectively, indicating that G x E interactions in the form of re-ranking of families across environments were relatively unimportant. The exception was Tas₁ at T₄, where the estimated correlations ranged from 0.11 to 0.46. The data were then combined and genetic correlations between family means at different time points were estimated. For T₁ onwards, the estimated correlations were 0.96 or greater, indicating that weight is effectively the same trait at all time points between T₁ and T₄. Correlations of average animal weight at T₀ with measurements at later time points were slightly lower, but still strong, ranging from 0.79 to 0.97. This indicates that measurement regimes based on early measurements could increase the efficiency of the breeding program. It was concluded that G x E interactions for weight will not be detrimental to the breeding program for this population, provided appropriate models for genetic evaluation of families are used.