The slugs Arion lusitanicus and Arion rufus inhabit ecologically degraded areas and are serious vegetation pests. In recent years, new localities of these species have been found in various parts of Poland. Here we study themorphology of 90 specimens from9 populations of slugs. The morphology of the genital system allowed for the identification of 60 A. lusitanicus specimens from 6 populations and 30 A. rufus individuals from another 3 localities. In order to describe their genetic diversity at the level of the individual, population, and species, we compared sequences of the mitochondrial cytochrome oxidase subunit I (cox1) gene. The morphological analysis revealed that each of the studied populations comprised a single species, which was also confirmed by the molecular assay.We obtained 674-bp sequences of the cox1 gene for each species that showed a total of eight haplotypes. The genetic diversity of A. lusitanicus individuals ranged from 0.5% to 2.1%, whereas that of A. rufus was twice as low: 0.4-1.0%. The difference between the two species within the cox1 gene was at the level of 12%. Three A. lusitanicus and two A. rufus populations were found to be monomorphic. Large inter-population variability was found within each of the studied species, which suggests that the Polish populations of A. lusitanicus may have originated from repeated, separate introductions arriving from various parts of Europe.