The influence of health-related physical fitness on age differences in vigilance as a function of the level of task complexity was examined in a group of 24 younger women aged 21-30 years (M = 24.04, S.D. = 2.51), and 23 older women aged between 61 and 83 years (M = 68.70, S.D. = 6.38). In the high event rate vigilance task, task demands were manipulated by degrading stimuli by 10, 20, and 30%. Age variation in performance was found in respect to time-on-task and level of stimulus degradation. As predicted, physical fitness moderated that age variance. It is concluded that physical fitness has greater influence on age differences in vigilance in situations placing high demands on attentional resources. The theoretical implications of the findings are discussed with emphasis upon the possible mediating role of the frontal cortex.