Existing studies of the effect on infant temperament of the 48 base pair variable number of tandem repeats polymorphism in exon 3 of the dopamine D4 receptor gene, DRD4 VNTR, and the serotonin transporter-linked polymorphic region, 5-HTTLPR, have provided contradictory results, and age seems to be an important factor. The present study investigated the effect of these two polymorphisms on the stability of infant temperament between 4 and 9 months of age. Furthermore, the effect of a recently discovered single nucleotide polymorphism which modulates the 5-HTTLPR (rs25531) was investigated in relation to infant temperament. The study sample consisted of 90 infants, who were assessed by parental report at the two ages under consideration using the Revised Infant Behavior Questionnaire. It was found that infants carrying the 7-repeat allele of the DRD4 VNTR had higher levels of Negative Affect. Furthermore, there was an interaction between DRD4 VNTR and 5-HTTLPR genotype such that infants with the DRD4 VNTR 7-repeat allele and the highest expressing 5-HTTLPR genotype (LALA) had the highest level of Negative Affect. These effects were largely driven by scores on the Falling Reactivity scale. Genetic effects were stable across age. The results emphasize the need for developmental studies of genetic effects on temperament.