A comparative study of the anatomy and characterization of the DNA of the arionids described by Torres Mínguez (1925) in Cantabria (Spain): Arion cendreroi and A. fulvipes, and A. rufus and A. vulgaris (Gastropoda Pulmonata: Arionidae). - In this work a comparative study of the genital and DNA system between topotypes of the species that Torres Mínguez (1925) described in Argoños (Cantabria, Spain) and European specimens of A. rufus and A. vulgaris were carried out. The intraspecific variability of the genital system and the stimulating organ is verified, and it is demonstrated that the ligule has taxonomic value. The topography of the A. cendreroi genital system coincides with that of A. rufus s. str. of Europe, whereas the A. fulvipes is different from the known species. By mitochondrial DNA analysis, p-Distance and K 2-p between A. rufus specimens from the study areas and A. rufus specimens from France and Poland is 16%, whereas with specimens from A. ater of Lithuania and Portugal is 21%. A. fulvipes has a genetic distance greater than 23% in relation to the species of the subgenus Mesarion of the Iberian Peninsula and 30% with A. vulgaris. It is concluded that new molecular studies are necessary to define the A. ater / A. rufus complex. Provisionally we consider A. cendreroi a synonym of A. rufus, and that the molecular separation is not well defined. A. fulvipes is considered a good species. We admit that 12% is the threshold that marks the genetic separation between species of the genus Arion.