On many farms in Britain, the housing of pregnant sows in groups, principally in electronic sow feeder systems, is becoming an increasingly popular alternative to housing them individually in stalls or tethers. When sows are housed in large groups, particularly in "dynamic groups" where individuals are continually leaving the group to farrow and re-joining it after lactation, aggression between group members may become a problem. Aggression may give rise to social stress and injuries and may also have detrimental effects on production. In order to ensure good welfare and high production in group housing systems, it is essential to gain some fundamental knowledge as to how individual sows are affected by the social environment in which they live. This paper examines how a group of gilts responded to being introduced into an electronic sow feeder system and mixed with unfamiliar animals. Detailed data on behaviour, physiology and reproduction were collected for each individual in the group /in an attempt to examine whether the way in which individuals responded to the social environment had consequences for measures of their welfare and productivity.