The Lesser Antilles is a biodiversity hot spot but unfortunately human disturbance has taken its toll, causing dramatic population declines and even extinction of numerous endemic species. Nevertheless, today the rediscovery of previously thought extinct species is not uncommon. Often, old museum specimens and their original descriptions are the only information available for such species. The application of molecular phylogenetic relationships to extant species can help to elucidate pivotal information on their ecology and conservation. Erythrolamprus cursor is possibly an extinct colubrid racer from Martinique, currently classified as critically endangered on the International Union for Conservation of Nature Red List. Mitochondrial deoxyribonucleic acid sequences were obtained from four E. cursor specimens from the Muséum national d'Histoire naturelle (Paris) collections. All sequences recovered the same haplotype and the level of divergence between E. cursor and E. juliae, from the nearby island of Dominica, was lower than between other intraspecific distances within other Erythrolamprus. Furthermore, phylogenetic analyses confirm that these two species are sister taxa and share most recent common ancestry. We discuss that published ecological data available for the sister species (E. juliae) may help to elucidate information on this species' natural history, ultimately having important implications for a future conservation management program if E. cursor is to be found. We emphasize the urgent need to conduct an exhaustive survey on the supposed last population of E. cursor at Diamond Rock to establish the survival of this species there, to understand how it may have adapted to such an ecosystem, especially in sympatry of several introduced rodent species.