Mymica rubra (L.) is an invasive ant in the northeastern U.S. and Canada. It is a Palearctic north temperate ant species with a native range that extends from Ireland and Great Britain, through northern Europe to western Siberia. Populations can be found from as far north as the Arctic Circle and south to the Black Sea. We studied the population structure of this species in parts of its native and invaded range to test the hypothesis that unicolonial population structure characterizes invasive populations and multicolonial population structure is characteristic of native populations. We collected colony fragments across a linear transect bisecting discrete geographic populations. Eleven populations in the United Kingdom and three populations in Maine (USA) were sampled. Three measures were used to test intercolony relationships. We conducted behavioral aggression bioassays, extracted cuticular hydrocarbons, and scored microsatellite markers. We found that in its native range, M. rubra is predominately multicolonial (increasing aggression with increasing distance between colonies, Mantel test, p<0.05), although evidence for unicolonial population structure (low levels of aggression over all intercolony distances) was also found (2 of 11 populations). Unicolonial populations were not found in the invaded areas in Maine. Similarity of cuticular hydrocarbons was negatively correlated with aggression level and intercolony distance in multicolonial populations, but not in unicolonial populations. Genetic similarity, as measured by FST, were not consistently related to aggression or cuticular hydrocarbon signature.