Turkey production in recent years has become very competitive and highly specialized. Fast-growing strains of turkey broilers are usually housed in large buildings, which may contain 1000-25,000 birds at stocking densities up to 60 kg/m2. Under commercial rearing conditions, domestic turkeys are often aggressive towards ‘pen-mates’, and this may lead to serious injuries or even death. Restricted space, as a consequence of high stocking density might prevent the attacked bird from retreating from its opponent. This study examined the aggressive response in 10 groups of five familiar turkey toms, comparing behaviour to an unfamiliar conspecific that is introduced to two pens of different sizes. Groups were tested both in small and in a large pen for 30 min. In the small pen they observed significantly more aggressive pecks and threats to the introduced bird. In the larger pen introduced birds kept a greater distance from the group than in the small pen. The mean number of fights and chases per test was not different between the two pens. In summary the present study shows that in small groups of turkeys, an increase in floor space reduces the number of aggressive pecks and threats aimed at an introduced unfamiliar conspecific. It is concluded that one of the reasons for high injury rates in fattening turkey toms reared under commercial breeding conditions might be the restricted space due to high stocking density which prevents the birds from moving far enough away from attacking conspecifics to avoid injuries.