The Australian gidgee skink, Egernia stokesii, is gregarious and lives in stable social aggregations. In this study we investigated whether individuals can recognize group members and distinguish them from non-group members, and we investigated whether recognition is based on relatedness. In one experiment, ten groups of three or four unrelated subadult lizards had been maintained for two years. Lizards from these groups were each presented with stimuli from themselves, from a member of their own group, and from a conspecific member of another group. Two types of stimuli were used; a paper substrate which had absorbed lizard body secretions, and lizard scats. For each type of stimulus, lizards showed a significantly stronger response, as measured by number of tongue flicks and by time in contact, to the stimulus from the non-group member, than to the stimuli from themselves or from a group member. In a second experiment, there were eight 'related' groups each of three sibling lizards, and seven 'unrelated' groups each of three individual lizards from different litters. Again lizards responded more strongly to stimuli from non-group members than to stimuli from group members. In a third experiment, juvenile lizards responded as strongly to stimuli from non-group members if they were related or unrelated. Together, these results imply that individuals in this lizard species can discriminate between two other lizards depending on whether they do or do not belong to the same social group. This suggests that individual lizards have unique chemical signals, and that an important component of social cohesion, the ability to recognize group members, is present in this gregarious lizard species. The results also suggest that familiarity, rather than relatedness, explains recognition of group members.