To date, physical exercise is the only intervention consistently demonstrated to attenuate age-related declines in physical function. However, variability exists in seniors' responsiveness to training. One potential source of variability is the insertion (I allele) or deletion (D allele) of a 287 bp fragment in intron 16 of the angiotensin-converting enzyme (ACE) gene. This polymorphism is known to influence a variety of physiological adaptions to exercise. However, evidence is inconclusive regarding the influence of this polymorphism on older adults' functional responses to exercise. This study aimed to evaluate the association of ACE I/D genotypes with changes in physical function among Caucasian older adults (n = 283) following 12 mo of either structured, multimodal physical activity or health education. Measures of physical function included usual-paced gait speed and performance on the Short Physical Performance Battery (SPPB). After checking Hardy-Weinberg equilibrium, we used using linear regression to evaluate the genotype*treatment interaction for each outcome. Covariates included clinic site, body mass index, age, sex, baseline score, comorbidity, and use of angiotensin receptor blockers or ACE inhibitors. Genotype frequencies [II (19.4%), ID (42.4%), DD (38.2%)] were in Hardy-Weinberg equilibrium (P > 0.05). The genotype*treatment interaction was statistically significant for both gait speed (P = 0.002) and SPPB (P = 0.020). Exercise improved gait speed by 0.06 ± 0.01 m/sec and SPPB score by 0.72 ± 0.16 points among those with at least one D allele (ID/DD carriers), but function was not improved among II carriers. Thus, ACE I/D genotype appears to play a role in modulating functional responses to exercise training in seniors. © 2014 the American Physiological Society.