A recent study indicated that acute aerobic exercise improves cognitive flexibility in adults. The current study assessed age, habitual physical activity, and physical fitness as moderators of this improvement and examined whether the gains still exist an hour after the exercise session. The alternative-uses test, assessing cognitive flexibility, was administered individually to 20 older (age 63.67 ± 3.55 yr) and 19 young (age 23.9 ± 1.22) women before, immediately after, and an hour after a single moderate aerobic-exercise session. Results indicated significant improvement in cognitive flexibility in the older group immediately after the exercise but a decrease at the 1-hr follow-up. Further analysis indicated that physical fitness accounted for this improvement (R = –.622, p < .01). No such differences were observed in the young group. Further studies are needed to examine the duration of this effect, as well as the role of physical fitness as a moderator of it.