Using natural venom collected from the red imported fire ant, Solenopsis invicta Buren, we determined the dose of venom necessary to kill 50% of the workers (LD₅₀) of Argentine ants, Linepithema humile (Mayr), and several other ant species that the fire ant may encounter in California. Of the species tested, the Argentine ant was most susceptible to the venom. On a weight-adjusted basis the red imported fire ant was 330 times more resistant to the venom than the Argentine ant. In addition, the native southern fire ant, Solenopsis xyloni McCook, was significantly more susceptible to the venom than S. invicta. Formica perpilosa Wheeler was most resistant to the venom when the LD₅₀ was not adjusted for weight, requiring a dose 684 times higher than that of the Argentine ant. When we tested synthetic venom (solenopsin A, which is the main component of S. xyloni venom) against the Argentine ant, the LD₅₀ value was unchanged. However, both S. invicta and S. xyloni were ≈6 times more susceptible to the synthetic compound than to natural venom. Therefore, S. invicta venom is less toxic to fire ants while being just as toxic to Argentine ants as the simpler, more primitive venom of S. xyloni. Fire ant gaster flagging and venom dabbing applies enough venom to the Argentine ant to be lethal and may help explain the superior interference competition shown by the red imported fire ant.