Based on a synthesis of the literature on time of day and physical fitness effects on cognition, the current study examined whether physical activity moderated time-of-day differences in older adults' performance on a working memory task. Sedentary older adults' working memory performance declined significantly from morning to evening, whereas more active older adults performed similarly across the day. This interaction did not extend to performance on a simple reaction time task. A novel explanation based on the selective effect of mental fatigue on executive control processes is proposed.