Evidence documenting associations between 5-HTTLPR and parenting behavior led to testing the hypothesis that this polymorphism moderates the effect of the quality of environmental context on maternal sensitivity. Participants were 210 Portuguese mothers and their preschool children, recruited from the community. An index reflecting the quality of the environmental context was derived based on nine markers (e.g. single parenthood; parental education, economic difficulties, family conflict, maternal psychopathology). Maternal sensitivity was measured observationally. Maternal saliva was collected with OraGene kits for genetic analysis. Results revealed a gene-X-environment interaction, such that short-allele homozygotes proved more sensitive to the family context than long-allele carriers (i.e. sL/LL), displaying the highest and lowest levels of maternal sensitivity, depending on, respectively, low and high quality levels of the environmental context. Because even mothers carrying the long allele evinced similar responsiveness to the environmental context, but to a lesser extent, findings proved consistent with the weak differential susceptibility model of person-X-context interaction. Results are discussed in light of prior and related gene-X-environment findings.