The effects of mixing on agonistic behaviour and performance were studied in 90 individually fed pigs, grouped at a mean live weight of 23 kg by one of the following three methods: (1) six unmixed littermates; (2) three pigs from each of two litters; and (3) one pig from each of six litters.Unmixed animals (treatment 1) did not fight and the mode of fighting differed between treatment 2 and 3. Mixing decreased daily weight gain in the period 23 to 100 kg for barrows, but not for gilts. For gilts, food conversion efficiency was negatively correlated with the number of received attacks in the observation period (80 min after mixing), and for the barrows to fighting injury scores.