The data from this experiment suggests that the mixing of strange sows into groups can be done without adversely affecting fertility. The lack of overall effect on litter size and farrowing rate provides no impetus to support or refute arguments concerning group housing. However, we did note a reduction in farrowing rate when sows were grouped at 14 days after insemination. This suggests that there is a period during early pregnancy when sows are more sensitive to the stresses associated with mixing. Based on the present data and that in the literature, this period of sensitivity will be at a minimum from 10 to 14 days after mating. However, more work will be needed to accurately delineate the sensitive period. There is no clear advantage of boar presence on reducing aggression and stress among newly mixed bred sows.