Records from 9429 pigs raised in 353 grower groups in a commercial Australian piggery were analysed to determine whether grower-group characteristics affected daily gain and backfat of individual pigs. Individual and group effects as well as their interactions were tested for significance (P < 0.05) in a mixed model, with sire fitted as a random effect. Group characteristics affected average daily gain (ADG) more than backfat (BF). The proportion of males in a group influenced both traits significantly, as did the average number of full siblings. Groups with 10-30% of the opposite sex had the highest BF and a 21-30 g/day reduction in ADG compared with the highest-performing groups with less than 10% of males. Each additional full sibling per group increased ADG by 5.5 ± 1.60 g/day and BF by 0.12 ± 0.05 mm. Additionally, ADG increased by 9.8 ± 2.64 g/day per second of group mean flight time and by 4.5 g/day per 10% increase in the proportion of Duroc pigs per group. Group size affected ADG (linear and quadratic) and BF (linear); however, the effect on ADG was considerably larger during the warmer grower season. In commercial piggeries, it may be possible to optimise individual daily gain through the manipulation of grower-group characteristics. Advantages for growth rate arose from including a portion of a calmer line of pigs within groups, optimising the stocking density in warmer months and maximising the proportion of quieter, less fearful pigs in grower groups.