The slugs of Catalonia (NE Iberian Peninsula): the species of the genus Arion of Alejandro Torres Mínguez (Gastropoda: Pulmonata: Arionidae).— Between 1923 and 1927, Alejandro Torres Mínguez described a total of 12 species of the genus Arion, 8 of which in Catalonia. The specimens were provided by a network of naturalists, who sent him the slugs preserved in alcohol, accompanied by annotations. The descriptions of the species are obscure and incomplete. The specimens were deposited in the Museum of Zoology of Barcelona. The objective of this work is to know the taxonomic validity of these nominal species. The types and topotypes were examined for this study. From 1989 to 2016, the typical localities were sampled on several occasions, and hundreds of topotypes were collected. For the identification of the species, data from the external and internal anatomy of the genital system were used. Particular attention was paid to the variability of species in terms of space and time. Molecular data were used to construct a phylogenetic tree based on the mitochondrial ND1 gene. Based on this anatomical study and DNA data, we reach the following conclusions: Arion rufus (Linnaeus, 1758) var. Draparnaudi Kolen is the ocher form of Arion magnus. Arion hortensis Férussac, 1819 var. chloridiscolor corresponds to a juvenile individual of A. magnus from la Garrotxa. Arion ruginosus, Arion collominiato, Arion nigrachlamydae, Arion nuriae and Arion lineispede are juvenile or adult individuals with different somatic coloration of A. magnus from Setcases, la Garrotxa or Vall de Núria in Girona. Therefore, all of them must be considered junior synonyms of A. magnus. All the citations of A. rufus, A. ater and Arion lusitanicus in Catalonia are doubtful, and it would be necessary to revise them all based on detailed anatomical studies.