Data from 508 Caucasian children in the NICHD Study of Early Child Care and Youth Development shows that the DRD4 (but not 5-HTTLPR) polymorphism moderates the effect of child-care quality (but not quantity or type) on caregiver-reported externalizing problems at 54 months and in kindergarten and teacher-reported social skills at kindergarten and first grade—but not thereafter. Only children carrying the 7-repeat allele proved susceptible to quality-of-care effects. The behavior–problem interactions proved more consistent with diathesis-stress than differential-susceptibility thinking, whereas the reverse was true of the social-skills' results. Finally, the discerned Gene × Environment interactions did not account for previously reported parallel ones involving difficult temperament in infancy.