Dugesia anceps(Kenk, 1930), formerlyPlanaria dubiaBorelli, 1895, is a poorly known species from southern South America (Paraguay and Argentina). In this paper an anatomical as well as histological description is made and the previous identifications by Böhmig (1902) and Hyman (1959) are discussed. It can be distinguished from the other Neotropical species by the following characters: the ventral testes; the anteriorly curved spermiducts, ending in two intrapenial sperm vesicles that run downwards and unite at the base of the penis papilla to form a narrow ejaculatory duct; the long penis which has an ample male atrium; a relatively small copulatory bursa and a short but well defined female atrium with infranucleate epithelium.Dugesia ancepsis the most abundant species of triclads in the mesosaprobe streams of southern Buenos Aires Province (Argentina).