The populations and distribution of two invasive ant species, fire ants, Solenopsis invicta and Argentine ants, Linepithema humile, in their shared expansion habitat is affected by their competitive interactions. Laboratory studies were conducted to determine aggression demonstrated by fire ants and Argentine ants during interactions. Intraspecific nestmate and non-nestmate interactions were observed for both fire ants and Argentine ants. Aggression, role of the ant (initiator or respondent), and mortalities were studied. When equal numbers of Argentine ants and fire ants interact, Argentine ants face a higher mortality risk; Argentine ants were often the initiators. Lab-reared fire ants were less aggressive toward each other than toward field-collected fire ants and Argentine ants. Field experiments with varying numbers of each species were conducted to explain shifts in invasive species populations. Previously Argentine ants were the major ant pests in Louisiana, and presently fire ants are the major ant pests in the area. No environmental factors were discovered which would explain this shift, however the aggression bioassays offer an explanation for this shift. When equal numbers of fire ants and Argentine ants interact, Argentine ants face a higher risk of mortality. The loss of intraspecific aggression and the ability Argentine ants have to build supercolonies does not appear to be enough of a competitive advantage to displace large populations fire ants.