Abstract: Arms races between predators and dangerous prey can lead to rapid and elaborate counter-adaptation. Newts of the genus Taricha possess the sodium channel blocker tetrodotoxin (TTX), which is lethal to most predators. Garter snakes in the genus Thamnophis have repeatedly evolved resistance to TTX through their ecological interaction with toxic newts. We examine the traits at the center of this predator-prey interaction and their diversification throughout the geographic range of the interaction. By exploring geographic patterns of phenotypic mismatch, larger scale arms-race dynamics are apparent, in which the predator sometimes appears to 'escape' the coevolutionary interaction. The molecular basis of TTX resistance in snakes may explain this one-sided result, because one or a few amino acid substitutions confer large magnitude changes in physiological resistance. Comparisons of sodium channel genes among populations and species of garter snakes reveals that de novo mutations lead to convergent molecular evolution in this normally highly conserved family of genes.