The single-island endemic O‘ahu tree snail Achatinella mustelina Mighels, 1845 is an endangered species with dimorphic shell chirality, persisting in small populations restricted to upper-elevation native forest in the Wai‘anae Mountains. We used an intraspecific molecular phylogeny (n = 21 populations) to evaluate the validity of subspecies, most of them introduced by Welch in 1938 on the basis of shell characters, by determining whether the nominal subspecies examined correspond to detectable molecular partitions and to examine the possibility that opposing shell chirality acts as a reproductive isolating mechanism. We mapped the nominal subspecies and shell chiralities onto a mitochondrial DNA (mtDNA) phylogram based on 86 cytochrome c oxidase I gene fragments and the extant range of the species. Although clear genetic breaks and haplotype clusters with well-defined boundaries exist and correspond to topographic features, each of the five monophyletic clades in the gene tree contains multiple supposed subspecies, haplotypes are shared between different subspecies, and none of the 13 nominal subspecies exhibits monophyly. Furthermore the mtDNA clades in the gene tree do not correspond to observed patterns in shell chirality, and both chiralities occur in all clades. Thus, the subspecies are not taxonomically valid and have no relevance for conserving genetic diversity, and chirality differences do not appear to impart a reproductive barrier in this species. Therefore, all subspecies of A. mustelina are herein synonymized.