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The terrestrial slug Arion vulgaris has recently spread across most of Europe, often causing the local extinction of resident populations of Arion ater s.l. The species hybridize, which leads to the prediction of massive introgression of A. ater genes into A. vulgaris. To test this, we used 16 microsatellite markers applied to samples of both species collected around Görlitz, Germany, during the invasion. Amongst A. vulgaris individuals with typical genitalia, an analysis using structure suggested that only 6% were appreciably admixed with local A. ater ; admixture did not increase over the course of the invasion. Amongst the ~4% of slugs with intermediate genitalia, microsatellites confirmed that they were often hybrids, their anatomy correlating with the estimated share of ancestry from each species. The microsatellites also distinguished the three subspecies of A. ater previously recognized on the basis of genital anatomy and mitochondrial DNA. The subspecies were not well mixed spatially, with A. a. ater in wilder places and A. a. rufus never found in the Polish part of the town; nevertheless, hybridization between them was occurring. Unexpectedly, the microsatellites indicated three genetic races amongst A. vulgaris ; these occurred in different districts and are mixing spatially and genetically only slowly. [ABSTRACT FROM AUTHOR]