Myrmica rubra L. was introduced into New England in the early 20th century, and at present, has a patchy distribution in parts of northeastern North America, including records from 31 communities in Maine. M. rubra is highly polygynous, and colonies reproduce vegetatively, forming dense local populations where conditions are favorable. Using mobile nests and baited arenas in a series of field aggression bioassays, we tested patterns of internest tolerance within and among local populations on Mt. Desert Island, ME. We found that foragers originating from fragments of the same colony or from neighboring nests retained a high level of intraspecific tolerance over several months, whereas significant intercolony aggression among workers was present between colonies within the same local patch separated by ≈10 m. Within populations, aggression score values were found to increase linearly with internest distance within a site. Aggression was highest between colonies from spatially different populations on the island and was higher still when nests were assayed against colonies at an off-island site 70 km away in Castine, ME. These data strongly suggest a multicolonial organization within and among local populations of M. rubra in parts of its introduced range. These findings contradict the loss of intraspecific aggression and unicolonial social structure over large geographic areas that have previously been observed in other invasive ant species, particularly Linepithema humile Mayr.