The Hawaiian Islands have long been seen as relatively devoid of native toxic plant species, a result of the paucity of herbivorous predators in this biogeographically isolated archipelago. This assertion has been subjective, without quantitative comparison to floras that evolved in the presence of such predators. We test this assertion by comparing the richness of toxic species in the native flora of the main Hawaiian Islands to that in the naturalized flora of these islands. That test shows the assertion to be correct: the relative abundance of toxic plant species in the naturalized Hawaiian flora is nearly ten times greater than the relative abundance of those species in the Hawaiian native flora. Of the approximately 150 toxic plant species now found in these islands, 92% are naturalized. The species richness of both native and naturalized Hawaiian plant species correlates positively with island size, as does the richness of toxic native and toxic naturalized plant species. These results, and the lack of significant residuals in species–area relationships across the main Hawaiian Islands, suggest that the relative youth of Hawai‘i Island is not needed to account for native plant species richness on this island.