The study of physical activity and cardiorespiratory fitness influences on cognitive and behavioral functioning has grown in interest over recent decades. The beneficial effect of exercise and fitness on cognition appears to be a robust phenomenon, with a stronger relationship to selective aspects of cognitive function involving extensive amounts of executive control. However, to date, the influences of an acute bout of exercise and cardiorespiratory fitness on indices of action monitoring have not been examined. PURPOSE: To study the influences of cardiorespiratory fitness and acute aerobic exercise on the behavioral and neuroelectric indices of action monitoring using a flanker task paradigm. METHOD: Twenty-eight (14 female) adults (18–22 years) were recruited based on maximal oxygen uptake (VO max) achieved during a graded maximal exercise test (GXT) such that an equal number of participants were placed into gender-balanced higher- and lower-fit groups. The error-related negativity (ERN) component of a response-locked event-related brain potential and behavioral measures of response speed, accuracy, and post-error slowing were measured during the completion of a flanker task following a 30-minute acute bout of aerobic exercise on a treadmill and following a 30-minute rest period for all participants. RESULTS: Results indicated that higher-fit adults exhibited increased post-error response slowing compared to lower-fit adults. No such group effects were observed for response accuracy. ERN findings replicated previous action monitoring studies with larger amplitude following error, relative to correct, trials at the FCz electrode site. Further, higher-fit adults exhibited reduced ERN amplitude compared to lower-fit adults. No such effects of acute aerobic exercise were evident for ERN amplitude or behavioral measures of action monitoring. CONCLUSION: These findings suggest that cardiorespiratory fitness exerts a beneficial influence on both behavioral and neuroelectric indices of action monitoring by increasing top-down executive control while acute aerobic exercise does not produce a meaningful effect on these measures. Supported by: NIH/NIAAG21188 (PI: Hillman)