Myrmica rubra (Linnaeus, 1758) is considered an invasive ant in North America. The genetic relatedness and its dispersive abilities were investigated using microsatellite analysis of colonies on the island of Newfoundland, Canada. The genetic diversity of Newfoundland M. rubra was low, likely as the result of founder effect. Colonies located near each other (< 1 km) generally had low pairwise F_ST values, which we interpret as considerable mixing of colonies, while populations that are spatially separated showed higher F_ST values and very little mixing. Aggression analysis was also used to determine the relatedness of populations. In this analysis, considerable aggression was observed among M. rubra worker ants from the different localities, and it depended on the level of genetic relatedness. Calculation of Pearson correlation coefficients showed a positive correlation in aggressive behavior with genetic relatedness. The molecular and aggression analyses of M. rubra populations from Newfoundland localities indicate that these ants are not unicolonial over large areas but supercolonial with colonies genetically similar and with aggression reduced toward con-specifics at the local scale (< 1 km).