Many North American producers are anticipating a change to group housing for sows. The overall purpose of this study was to determine how to reduce the stress of mixing sows by altering space allowance, and social groups. We also studied how space can influence behaviour and aggression within a goup. The largest space requirement occurred between midnight and 8am when the highest percentage of sows were lying laterally. From a purely physical perspective the sows would require 1.51m2/sow, however this does not account for any movement or interactions between individuals. When sows were initially grouped, they showed a higher occurrence of injury scores (P<0.001) and a greater number of fights (P<0.001) compared to the stable groups (3 weeks post-mixing).  Most fi ghting and injuries occurred within 24 hours of mixing. There was not a significant difference between either injury score and number of fights with the different space allowances. Passive/shy sows appeared to show a reduced stress response compared with active/bold sows.