Aggressive encounters and injuries due to head pecking seriously threaten the welfare of domestic turkeys and also result in economic losses for the turkey industry. Animals of different species show an increase in aggression when unfamiliar conspecifics are mixed but a marked drop in aggression occurs as they become familiar with each other. This experiment was conducted to investigate the effect of group size on the frequencies of different types of aggressive interactions performed by 12-week-old broad-breasted turkey toms. Non-group members were introduced for 30 min into 12 small (6 bird) and 12 large groups (30 birds) and the subsequent aggressive behaviour was observed. The members of small groups initiated more fights and delivered more aggressive pecks to the introduced bird than members of larger groups. Also birds originating from small groups initiated more fights and delivered more aggressive pecks than birds from large groups. This study demonstrates that groups of turkey toms react differently to an introduced conspecifics depending on their group size.