Genetic by environment (GxE) interactions were estimated for harvest traits of Pacific oysters Crassostrea gigas based on data collected over a period of 20 y from farm trials conducted by the Molluscan Broodstock Program, a family-based selective breeding program aimed at improving yields of Pacific oysters on the West Coast, USA. Genetic by environmental interactions were estimated for oyster families planted at subtidal and intertidal sites, as well as at coastal and Puget Sound (PS) sites in the Pacific Northwest. Covariance components were obtained using a multiple-trait mixed model, treating one type of trait in different environments as two different but correlated traits. A total of 9,970 records were analyzed for five generations of selectively bred oysters. Heritability estimates for yield and average individual weight at harvest (total weight of all living oysters in a container divided by the number of live oysters) were similar in both inter- and subtidal environments, ranging from 0.42 ± 0.02 to 0.45 ± 0.02; however, the heritability for survival was higher (0.45 ± 0.02) in intertidal than in subtidal environments (0.31 ± 0.02). Genetic correlations among harvest traits within environments ranged from small-negative (-0.05 ± 0.06) to high-positive (0.77 ± 0.02) values. Genetic by environmental correlations were moderate to high between inter- and subtidal environments (0.66 ± 0.04, 0.81 ± 0.05, and 0.64 ± 0.06 for yield, survival, and average individual harvest weight, respectively), whereas correlations between coastal and PS environments were moderate (0.46 ± 0.06, 0.69 ± 0.05, and 0.52 ± 0.06 for yield, survival, and average individual harvest weight, respectively), indicating that GxE interactions were stronger between the coastal and PS sites. Indirect responses to selection were mostly lower compared with direct selection. Stronger GxE effects and low indirect responses to selection for yield between coastal and PS sites suggest that developing separate breeding programs, rather than a single breeding program, would be more effective in improving harvest-related traits of Pacific oysters farmed in these two different geographic regions.