The aggressive behaviour of free-ranging guanacos and vicunas, living in sympatry in Catamarca Andes, Argentina, was analysed in relation to their social organisations. These organisations include territorial family units, solo males (territorial or not), and bachelor groups. In both guanacos and vicunas, family groups tended to have higher rates of agonistic interactions than bachelors and to be frequently involved in aggressive encounters, whereas bachelor groups showed the opposite tendencies. Encounters between family groups and solo males were more frequent than expected. Bachelors acted almost exclusively as recipients of aggressive interactions, whereas solitary and family males frequently initiated encounters. Family groups adopted different behaviours towards bachelors in the two camelids. In guanacos, the latter were simply ignored, whereas in vicunas (where they tend to compete for food with families) bachelors were consistently attacked by families. The behaviour of solitary males showed the main difference between the two species. Guanaco solo males had a much more aggressive attitude than vicuna counterparts. Their proportion in guanaco population was also unexpectedly high. These results suggest that, in the less rigid organisation of guanaco society, the behaviour of solitary males is adopted by a number of adult males in an attempt to breed.