Genetic factors may partly account for between-person differences in brain integrity in old age. Evidence from human and animal studies suggests that the dopaminergic system is implicated in the modulation of white-matter integrity. We investigated whether a genetic variation in the Catechol-O-Methyltransferase (COMT) Val158Met polymorphism, which influences dopamine availability in prefrontal cortex, contributes to interindividual differences in white-matter microstructure, as measured with diffusion-tensor imaging. In a sample of older adults from a population-based study (60-87 years; n = 238), we found that the COMT polymorphism affects white-matter microstructure, indexed by fractional anisotropy and mean diffusivity, of several white-matter tracts in the oldest age group (81-87 years), although there were no reliable associations between COMT and white-matter microstructure in the two younger age groups (60-66 and 72-78 years). These findings extend previous observations of magnified genetic effects on cognition in old age to white-matter integrity.