Previous studies indicate that low resting heart rate is probably the best-replicated biological correlate of childhood antisocial and aggressive behavior. Nevertheless, there have been few longitudinal tests of this relationship, little control over potential confounds and mediators, and no test of its cross-cultural generalizability. This study tests the hypothesis that low resting heart rate at age 3 years predicts aggression at age 11 years. Resting heart rate at age 3 years was assessed in 1,795 male and female children from Mauritius. Aggressive and nonaggressive forms of antisocial behavior were assessed at age 11 years using the Child Behavior Checklist. Aggressive children had lower heart rates than nonaggressive children (p < .001). Conversely, those with low heart rates were more aggressive than those with high heart rates (p < .003). There were no interactions with gender or ethnicity. Evidence was found for specificity of low heart rate to aggressive forms of antisocial behavior. Group differences in heart rate were not attributable to 11 biological, psychological, and psychiatric mediators and confounds. It is concluded that low resting heart rate, a partly heritable trait reflecting fearlessness and stimulation-seeking, is an important, diagnostically specific, well-replicated, early biological marker for later aggressive behavior.