The mixing of unfamiliar sows usually results in fighting which determines relative social rank. In some situations aggression can be perpetuated and subordinate animals can suffer as a result of competition for resources such as feed and space. Although Moore et al (1993) suggested that newly introduced group members become fully integrated after 21 days there have been few studies which have examined the development of social interactions over the longer term. The aim of the experiment was to determine how social behaviour changed over time and the affects that this may have on the welfare of group-housed sows.