Most UK pig farmers will be turning to housing sows in groups due to the regulations which will phase out stall and tether systems by 1st January 1999. The formation of either small stable groups or large, dynamic groups by mixing unfamiliar sows, will result in aggression between sows. One consequence of aggression is the level of damage to the skin which, if severe, may compromise the welfare of the sow. Information is needed on the effect of different methods of mixing and managing sows in groups on the level of skin damage and, hence, the welfare of the sow. In addition, aggression between sows in early pregnancy may have deleterious effects on reproductive performance. In particular, Simmins (1993) showed that sows housed in stable groups throughout the implantation period of pregnancy had larger, heavier litters than sows housed in dynamic groups. However, no information was available on the level of aggression between sows on each treatment. The object of the present experiment was to determine the level of skin damage, as an indicator of aggression between sows, that occurs when sows are mixed in small stable groups, before or after mating, or large dynamic groups after mating.