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The fire ants, Solenopsis invicta and S. richteri, were accidentally imported into the United States in the first half of this century from South America. In their adopted habitat the imported fire ants have thrived causing considerable medical and agricultural problems in the nine widely infested states of the south and southeast. The red imported fire ant, S. invicta was considered the dominant ant in the infested areas, having displaced the black imported fire ant, S. richteri, into a small enclave in north- eastern Mississippi. However, a large reproductively viable S. invicta/S. richteri hybrid population was recently discovered across northern Alabama and into Mississippi and Georia by chemical analysis. This paper reports on the use of three species-specific chemical characters (venom alkaloids, cuticular hydrocarbons, and trail pheromones) to define S. invicta, S. richteri, and hybrid populations in the United States. In addition, these characters have been applied to fire ant taxonomy in South America. We also discuss fire ant population dynamics in the United States and its implications on several models of hybridization. These results have important consequences regarding the species status of the two imported fire ants and the taxonomy of fire ant populations in South America.