Datasets underlying the article: One town’s invasion by the pest slug Arion vulgaris (Gastropoda: Arionidae): microsatellites reveal little introgression from Arion ater and limited gene flow between infraspecific races in both species J.M.C. Hutchinson, B. Schlitt, H. Reise Published online 2021 in Biological Journal of the Linnean Society: https://doi.org/10.1093/biolinnean/blab082 Data_explanations.txt explains the contents of the other two files. Microsat_data.csv is a tab-separated file, formatted to load into Structure; for each of 218 slugs it lists the microsatellite allele lengths from 16 loci. Other_data.csv is a tab-separated file which lists further attributes of each slug; it includes also details of which individuals were included in each Structure analysis and the estimated ancestries calculated by these analyses.