The black-lipped pearl oyster, Pinctada margaritifera, produces the largest and most valuable coloured pearls in the world. Cultured pearl size remains one of the most important quality traits. Despite the great geographical area covered by pearl farms in the atolls of French Polynesia, little is known about the influence of grow-out site effects on pearl size attained. To explore the genetic and environmental impact on the size of pearls as well as the genetic × environment interaction, a uniform experimental graft was designed on two contrasting macro-geographical lagoons. Five biparental families of donor oysters were grafted and then reared at both sites. After 18 months of culture, phenotypic parameters corresponding to pearl size, i.e. nacre weight and thickness, were recorded among the harvested pearls. The expression of Pif-177 gene, a biomarker encoding protein in the aragonite nacreous layer, was analysed in the corresponding pearl sac. The results show a family effect for nacre weight and thickness on both sites, with family F058 producing the heaviest and thickest nacre, and F805 the lightest and thinnest. By contrast, inter-site comparison revealed no significant site effects for these two parameters. In addition, grow-out location did not modify the relative gene expression of Pif-177 in the pearl sac between donor families in either culture site. Both nacre weight and thickness were positively correlated with the level gene expression of Pif-177. These results suggest that pearl size parameters were not affected by the environment in the present study and this is supported by the relative gene expression of Pif-177 observed. This knowledge constitutes an initial step in the study of pearl size trait inheritance, which will be helpful in the near future for the diffusion of genetically selected donor oyster lines produced by hatchery systems throughout production sites.