The authors examined task-based (i.e., executive function), surveyed (i.e., effortful control), and physiological (i.e., resting cardiac respiratory sinus arrhythmia [RSA]) measures of child and maternal regulation as distinct moderators of longitudinal bidirectional links between child externalizing (EXT) behaviors and harsh parenting (HP) from 6 to 9 years. The sample size was 299 (50.9% female; 1% Asian, 4% multiple races; 14% Black; 78% White), and participants were recruited in the United States (a rural college town in Virginia and a midsized city in North Carolina). Higher child EXT at 6 years predicted higher HP at 7-8 years, which predicted higher EXT at 9 years. Also, this path was moderated by 6-year child effortful control, 6-year resting RSA, and 9-year executive function. In contrast, there was no moderating effect of any measure of maternal regulation. Findings suggest it is important to consider child self-regulation when examining bidirectionality in parent and child effects for HP and child EXT.