The slugs of the genus Deroceras (Mollusca, Gastropoda, Agriolimacidae) include more than one hundred species. One of them, D. laeve, has a worldwide distribution, a fact that could be related to its biology and life history. This slug has an economic and sanitary importance, as it is an intermediate host of parasitic nematodes, causes crop losses, and can also affect the native fauna. In this study, we carried out a phylogenetic and phylogeographic study of D. laeve based on the cytochrome oxidase subunit I marker (COI), considering specimens collected in Buenos Aires (Argentina) and sequences deposited on online platforms. This information was integrated with anatomical dissections of the radulae, maxillae, and genitalia. As a result, we re-identified other species of Deroceras, found that D. laeve consists of a species complex of at least six operational taxonomic units (OTUs), and showed statistically that COI can be regarded as a good marker delimiting species within the D. laeve complex. In addition, possible distribution pathways of the most diversified OTU are discussed, and previous anatomical studies and radular formulas are compared with those found in the present study. In order to contribute to the knowledge of the D. laeve complex, we encourage further studies integrating molecular, anatomical and ecological results.