Maternal sensitive responsiveness and extreme insensitivity only partly explain the variance in attachment security. Differences in attachment security may well be rooted in the interplay of genetic variations and environmental factors. The association between parenting (observed sensitive responsiveness and extreme insensitivity) and attachment security (assessed with the Strange Situation Procedure) was hypothesized to be moderated by genes involved in the regulation of the stress response: the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) genes. A significant G. × E interaction was found: infants carrying the minor MR allele (G) were significantly more securely attached if their mothers showed more sensitive responsiveness and significantly less securely attached if their mothers showed more extremely insensitive behaviors. These associations were not significant for carriers of the AA genotype of MR. Findings are discussed from a differential susceptibility perspective.