Relational aggression is linked to numerous adverse consequences. However, we know little about how temperament leads individuals to become perpetrators/victims of relational aggression, or how being a perpetrator/victim influences the development of temperament. We used longitudinal data from 674 Mexican-origin youth to examine relations between relational aggression and mother- and child-reported temperament from 5th grade (Mage=10.8; SD=0.60) through 11th grade (Mage=16.8; SD=0.50). Results show that: (a) high Negative Emotionality and low Effortful Control predicted increases in victimization; (b) low Effortful Control predicted increases in perpetration; (c) victims increased in Negative Emotionality and decreased in Effortful Control; and (d) perpetrators increased in Negative Emotionality and Surgency. Thus, temperament serves as both an antecedent to and a consequence of relational aggression.