It is well known that a gender difference in physical aggression emerges by the preschool years. We tested the hypothesis that the gender difference is partly due to changing tactics in peer interaction. Observations of girls’ and boys’ social initiatives and reactions to opportunities for conflict were made, using the Peer Interaction Coding System (PICS) in four independent samples of children between 9 and 36 months of age, which were aggregated to form a summary data set (N= 323), divided into two age bands (below or above 24 months of age). Linear mixed-model analyses revealed significant age by gender interactions in the use of bodily force in response to peers’ initiatives and in the tendency to use bodily force at later stages of conflicts with peers. The gender difference in use of force was not explained by differences in the use of verbal tactics. These cross-sectional findings suggest that girls are initially more likely than boys to use reactive aggression, but then desist, whereas boys increase their use of force to defend their territory and possessions. The difference between older and younger girls likely reflects girls’ abilities to regulate their behaviour in response to social challenges and the fact that girls are explicitly socialized to yield to peers’ demands.