<p>Animals often rely on kin discrimination to direct altruism towards relatives. Ants are a well-known example, because they ordinarily target aggression towards non-nestmate conspecifics. Many ant species can form multi-queen nests, in which relatedness between nestmates may be very low, raising questions about their origin and maintenance under inclusive fitness logic. Using the socially polymorphic ant <i>Formica exsecta</i> we tested if workers from single queen and multi-queen nests differed in discrimination against young queens of different origin in experimental bioassays. We also studied, for the first time, how workers respond to males from different origins. Together these also allowed testing whether outsiders of either sex were accepted as potential mating partner for residents. Workers from both single queen and multi-queen nests show high levels of aggression towards individuals from foreign colonies, but workers in multi-queen nests that are parts of multi-nest colonies readily accept both young queens and males from other nests within their own multi-nest colony. Discrimination by workers was not complete as some foreigners were occasionally accepted. This was not, however, dependent on the presence of potential mating partners in recipient nests, suggesting such acceptance was erroneous rather than adaptive. Our results suggest that differences in social organization between single queen and multi-queen nests are not associated with differences in nestmate discrimination. The causes and consequences of discrimination errors warrant further long-term studies.</p>