Recent studies have shown that mixed gender and same gender sibling dyads participate more frequently in different forms of play (Stocker, 1994,Vespo et aI, 1995). Studies have also shown that rough and tumble play has a significant effect on a child's learning of aggressive behaviours. (Patterson et aI, !989, 1992; Pellegrinni & Smith, 1998 ). This study aimed to investigate whether the gender of the elder sibling had a significant impact on the younger male siblings participation level in rough and tumble play and the later developed levels of aggression. Data was collected using questionnaire-based measures of play behaviour and levels of aggression. Males with elder male sibling were expected to have significantly higher levels of participation in rough and tumble play and significantly higher levels of aggression. Results were analysed using multiple independent sample t-tests. A significant difference was found in participation levels of rough and tumble play and three of the five levels of aggression. Explanations of these results were offered along with implications regarding a better understanding of plays relation to aggression. The possible use of monitoring play within schools and family settings was discussed. Methodological and theoretical issues were also addressed.