Unicoloniality is thought to be responsible for the invasion success of introduced populations of the Argentine ant, Linepithema humile. Fusion of unrelated colonies may be a mechanism involved in the formation of expansive colonies. We determined whether colony pairs displaying high, intermediate, and low levels of aggression would fuse. For each pairing, we measured the number of workers fighting, the number of dead workers, and whether colony fusion occurred. Many workers fought and were killed in the highly aggressive pairs, and these did not merge. Fewer workers fought with low to moderate mortality in colony pairs with intermediate levels of aggression and these merged only in a few cases. All low aggression replicates merged within 4 h. Final colony size (workers, brood, and queens) after 6 months was considerably larger for colonies that merged vs. colonies that did not merge. After 6 months, levels of aggression remained low or even decreased in colony pairs that merged, while high levels of aggression were maintained between colonies that did not merge. Cuticular hydrocarbon and genetic analyses will provide further information to better understand the dynamics involved in colony fusion.