Plagiodontes daedaleus (Deshayes, 1851) is the most widespread species in the genus Plagiodontes Doering, 1877. Although some 19th century authors recorded some differences in shell shape between the populations living to the east or to the west of the Pampean Sierras, Argentina, such differences were ignored by later authors. A multivariate statistical analysis on 608 adult shells collected at 29 localities in and around these mountains was used to test if such narratively described shell differences were statistically significant, irrespectively of their taxonomic status. Shells classified as collected from the east or the west of the sierras were correctly identified in 91.9% of the cases by Multivariate Discriminant Analysis, leading to recognizition of two allopatric shell morphologies. These external divergences were found to correspond with anatomical differences in the reproductive system. We conclude that two different, allopatric species were merged under the name Plagiodontes daedaleus in the literature and raise the subspecies Plagiodontes daedaleus strobelii (Doering, 1877) to the specific level. Another subspecies living to west of the sierras, Plagiodontes daedaleus salinicola (Doering, 1877), also showed a simple penis papilla and a non-swollen vagina, which shows that salinicola is not a member of P. daedaleus but rather a population of P. strobelii that does not deserve subspecific status. The taxonomic significance of the presence of either a simple penial papilla or a papilla bearing an accessory lobe was also confirmed for these Plagiodontes species.