Laying hens kept in small groups exhibit social behaviour similar to that of their wild ancestors, red jungle fowl, with a dominance hierarchy which is most likely based on establishment fights. In larger groups this system is thought to break-down, and hens may adapt to the new environment with a different social strategy. The present study tested the ability of laying hens living in groups of 10 and 120 to discriminate flockmates from unfamiliar birds. Subjects were presented in test cages with vertical bars, positioned outside the home pens in one of two places. Sixteen subject hens from each group size were presented to their home group (familiar) and a different group of the same size (unfamiliar). Hens in small groups discriminated between familiar and unfamiliar subjects by showing more aggression towards unfamiliar hens. In large groups, the overall level of aggression towards subjects was reduced in that attempted fights were rare, and aggressive outcomes were no different in response to unfamiliar or familiar hens. Certain individual hens in large groups were more likely to approach the subjects showing no evidence that hens have limited ‘territories’ within the pen. Those birds approach and aggressive towards the subjects in large groups were heavier and had larger combs than the subjects. These findings are consistent with idea that in large groups hens become less aggressive and may change their social system to one where dominance is determined through direct assessment and ‘status signalling’ rather than the remembered individual assessment of a small group pecking order.