Canis familiaris is a highly social species. Among the social relationships between two individuals, a particular case is represented by attachment bonds. The aim of the current study was to assess whether the bond between adult cohabitant dogs may be regarded as attachment.Twenty-two couples of dogs living in the same household participated in the study. Each couple was tested through a modified version of the Ainsworth strange situation test, in which one dog (11 males and 11 females) was tested and the other one acted as the presumed attachment figure; the stranger was played by a 25 year old woman. As females and males behaved virtually in the same way, their data was combined.Dogs were found to show less signs of stress (whining and behaviours towards the door) in the presence of the cohabitant dog than alone; and dogs appeared less stressed in the company of the stranger (shorter duration of whining, close to the door and behaviours towards the door) than in isolation. Dogs also showed a higher contact maintenance effect towards the stranger, especially after reunion with her, compared to affiliative behaviours towards the other dog.The presence of an attachment bond between adult dogs was not fully supported by our results, although the presence of a cohabitant dog strongly diminishes dog stress response to isolation. Not surprisingly, a human stranger has a strong ameliorative effect. Further research is needed to better understand this important aspect of canine social behaviour.