Despite consistent evidence that serotonin functioning affects stress reactivity and vulnerability to aggression, research on serotonin gene-stress interactions (G x E) in the development of aggression remains limited. The present study investigated variation in the promoter region of the serotonin transporter gene (5-HTTLPR) as a moderator of the stress-aggression association at the transition to adulthood. Multiple informants and multiple measures were used to assess aggression in a cohort of 381 Australian youth (61% female, 93% Caucasian) interviewed at ages 15 and 20. At age 20, semistructured interviews assessed acute and chronic stressors occurring in the past 12 months. Structural equation modeling analyses revealed a significant main effect of chronic stress, but not 5-HTTLPR or acute stress, on increases in aggression at age 20. Consistent with G x E hypotheses, 5-HTTLPR short allele carriers demonstrated greater increments in aggression following chronic stress relative to long allele homozygotes. The strength of chronic stress G x E did not vary according to sex. Variation at 5-HTTLPR appears to contribute to individual differences in aggressive reactions to chronic stress at the transition to adulthood. (Contains 2 tables, 2 figures, and 3 footnotes.)