Attachment to the mother plays a key role in the psychobiological development of the young. Studies in primates have demonstrated that isolation and peer-rearing in infancy results in behavioural disturbances. In non primates however, the role played by the mother in the development of social interactions and social preferences is still obscure. This was investigated in sheep by comparing groups of lambs reared from birth with or without their mother. General activity and nearest neighbour were recorded via scan sampling at various times. After nearest neighbours were identified, a reunion-separation test was performed in a novel environment in order to assess attachment behaviours. Mothered subjects were sequentially reunited with and separated from their mother and another maternal ewe while peer-reared lambs were reunited with and separated from their nearest and furthest neighbour. A preference test was then performed where lambs had to choose between a group of 3 familiar and 3 non familiar congeners. Mothered lambs associated far closer with their mother than with any other ewe, and sometimes with specific juveniles. Even though some associations were observed occasionally in peer-reared lambs, they huddled mostly together in a single group when resting or displayed activities in a synchronised manner. In the reunion-separation test, while mothered lambs were strongly attracted to their mother during reunion, and distressed when separated (increased vocalisation and locomotion), they did not react to the other ewe. Peer-reared lambs by contrast displayed no specific reaction to any neighbour. In the preference test, mothered lambs spent significantly more time near the familiar than near the unfamiliar group of congeners. Such discrimination was not expressed by peer-reared lambs. In conclusion, social preferences are clearly influenced by the presence of the mother. As the primary attachment figure, she participates in her offspring’s socialization process.