A number of introduced ant species are characterized by an absence of nestmate recognition or discrimination. This unicolonial social behavior often aids these species in outcompeting native species to the extent of becoming serious urban and agricultural pests. The Argentine ant, Linepithema humile (Mayr), is a widespread pest species that is thought to rarely show intraspecific aggression in their introduced populations. We found, however, that colonies from both within Los Angeles and between Los Angeles and other cities in California will frequently exhibit aggression toward each other. When the aggressive colonies were retested after 2 mo under uniform conditions, all of the formerly incompatible pairs exhibited little to no aggression. Thus, intraspecific aggression does exist across introduced populations of Argentine ants, and nestmate recognition is likely to be based primarily on environmental cues. It may be possible to potentially manipulate such cues between neighboring nests to aid in the biocontrol of this pest species.