As red imported fire ant, Solenopsis invicta Buren, continues to expand its range into the southwestern United States, it can be easily confused with the native southern fire ant, Solenopsis xyloni McCook. Variability in the morphological characteristics commonly used to differentiate these ant species was quantified by examining the length of the clypeal tooth, striations of the mesopleuron, length of antennal scape, area of the petiolar process, number and size of mandibular teeth, and color by using both scanning electron and light microscopy. Given enough samples, the average values of each of these characteristics is different between the two species; however, significant morphological variability occurs in both S. xyloni and S. invicta populations, creating an area of overlap where either of the two species could exhibit similar characteristics. Better differentiation of these two species is achieved using a combination of characteristics, but morphological techniques are not dependable unless numerous ants from each population are analyzed by a taxonomist familiar with Solenopsis. For situations requiring a more accurate identification, such as before quarantining a county or a portion of a county, a molecular technique using mitochondrial DNA and polymerase chain reaction techniques was developed.