Cryptogenic species, those whose native and introduced ranges are unknown, pose a challenge not only to conservation but also to studies of ecology, biogeography, and evolution. One such species is the Lesser Antillean frog, Eleutherodactylus johnstonei, which is broadly distributed through the Caribbean and mainland South America. The Lesser Antilles pose a particular challenge with regard to cryptogenic species because these islands have been anthropogenically connected since before recorded history. We combined genetic (available on Genbank) and morphological data to infer the native and introduced ranges of this species. We inferred the insland of Montserrat as the native range of E. johnstonei.  Furthermore, we identified two major clades of E. johnstonei,  only one of which is responsible for the widespread introduced range. Based on previous suggestions that body size differed across the range of E. johnstonei, we collected snout-urostyle length from 98 females and 582 males throughout the range. We found that female body size did not differ between clades. Although maximum male body size did not differ, on avergae males were larger in the eastern clade comrpising the bulk of introduced localities. Our results demonstrate the utility of population and phylogeographic tools in resolving the cryptogenic species problem.