Associations between physiological reactivity to exclusion (i.e., heart rate [HRR], respiratory sinus arrhythmia [RSAR], and skin conductance [SCR]) and relational and physical aggression were assessed. It was hypothesized that blunted "fight or flight" responses to stress (i.e., blunted HRR, SCR, and RSA withdrawal) would be associated with relational aggression, whereas heightened "fight or flight" responses (i.e., heightened HRR, SCR, and RSA withdrawal) would be associated with physical aggression. In addition, it was hypothesized that heightened "fight or flight" responses would interact with social and cognitive risk factors in the prediction of physical aggression. Data were collected at an all-girls residential summer camp (mean age=12.47 years; N=119). Overall, findings indicated that blunted "fight or flight" was associated with relational aggression whereas heightened "fight or flight" was associated with physical aggression, particularly in the context of high social and cognitive risk. These findings contribute to our understanding of the relationship between physiological reactivity and different forms of aggression in girls.