Many distantly related insect species are specialized feeders of cardenolide-containing host plants such as milkweed (Asclepias spp.). Previous studies have revealed frequent, parallel substitution of a functionally important amino acid substitution (N122H) in the alpha subunit of Na(+) ,K(+) -ATPase in a number of these species. This substitution facilitates the ability of these insects to feed on their toxic hosts and sequester cardenolides for their own use in defense. Among milkweed butterflies of the genus Danaus, the previously established phylogeny for this group suggests that N122H arose independently and fixed in two distinct lineages. We reevaluate this conclusion by examining Danaus phylogenetic relationships using >400 orthologous gene sequences assembled from transcriptome data. Our results indicate that the three Danaus species known to harbor the N122H substitution are more closely related than previously thought, consistent with a single, common origin for N122H. However, we also find evidence of both incomplete lineage sorting and post-speciation genetic exchange among these butterfly species, raising the possibility of collateral evolution of cardenolide-insensitivity in this species group.