Prosocial behavior is an important aspect of normal social and psychological development. Adult and child twin studies typically estimate the heritability of prosocial behavior to be between 30 and 50%, although relatively little is known about genetic and environmental influences upon prosocial behavior in adolescence. We therefore examined reports of prosocial behavior in a large longitudinal family study of 1160 adolescent twin pairs (aged between 13 and 19 years). Prosocial behavior was assessed at two time points by self-report and at the second time point by additional parent-ratings using the Strengths and Difficulties Questionnaire (SDQ; Goodman, 1997). Adolescent females were reported to be significantly more prosocial than males (p < .001). Univariate analyses primarily showed moderate heritability and large nonshared environmental influences. There was a moderate genetic correlation between self- and parent-reported prosocial behaviour, suggesting that both types of rater were tapping into genetically overlapping constructs. Longitudinal analyses revealed that continuity was largely explained by genes. Unique environmental influences were predominantly time-specific and were the major source of individual differences.