Invasive species can impose novel selection pressures on natives, such as toxins to which native taxa are not adapted. Native species may survive such invasions by evolving mechanisms to avoid toxin exposure or increase toxin tolerance. Red imported fire ants (Solenopsis invicta) employ an alkaloid-based venom to defend their colonies and capture prey. In this study we aim to characterize the sublethal effects of invasive fire ant venom on a native vertebrate, the eastern fence lizard (Sceloporus undulatus), and to determine whether lizard populations that have been exposed to these fire ants for ∼35 generations have increased physiological resistance to the venom. We documented the sublethal impact of fire ant venom on fence lizard performance by naturally exposing lizards to fire ant stings and recording changes in three fitness-relevant measures: bite force, righting ability, and sprint speed. We also measured blood hemolysis induced by the venom. To test for the development of physiological resistance to fire ant venom we compared whole-body performance and hemolysis for two populations of lizards with different fire ant invasion histories. Fire ant venom showed no dosage-dependent sublethal effects on performance. In addition, there is no evidence that lizards have evolved increased physiological resistance: the impact of fire ant venom on whole-body performance and hemolysis did not differ between the naïve and experienced populations. Lizards may instead rely on adaptive shifts in escape behavior and morphology following invasion to survive fire ant attack. J. Exp. Zool. 313A:17–23, 2010. © 2009 Wiley-Liss, Inc.