Coping Power is an evidence-based preventive intervention for youth with aggressive behavior problems that has traditionally been delivered in small group formats, but because of concerns about potentially diminished effects secondary to aggregation of high-risk youth, an individual format of Coping Power has been developed. The current study examined whether physiological characteristics of the child may provide information about which intervention delivery format works best for that individual. Indicators of sympathetic and parasympathetic nervous system functioning were examined in 360 fourth-grade children (65% male; 76.4% self-reported African-American) who were randomly assigned to Group Coping Power (GCP) or Individual Coping Power (ICP) (Lochman et al. 2015). Longitudinal assessments of teacher- and parent-reported proactive and reactive aggression were collected through a 1-year follow-up. For children with higher initial levels of aggression, those with lower parasympathetic functioning at pre-intervention showed greater reductions in teacher-rated proactive aggression in the ICP condition than the GCP condition. For children with high parasympathetic functioning, there was no differential effect of intervention format. Regardless of intervention format, youth with lower levels of sympathetic functioning at pre-intervention demonstrated greater reductions in teacher-rated proactive aggression. These findings suggest that physiological indicators may be worth considering in future studies examining which youth respond best to specific types of interventions.