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The terrestrial slug Deroceras invadens has spread across much of the world over the last century. What is there to learn about its origin, colonizations and diversity by comparing barcoding sequences of the mitochondrial gene COI (cytochrome c oxidase subunit I)? Samples from 317 localities covering most of the species' range yielded 87 haplotypes. Higher diversity, the predominance of private haplotypes and geographic structuring all indicate a native range centred on southern Italy, including eastern Sicily. In contrast, central Italy is dominated by one haplotype, although accompanied by both close and more distant relatives; the lack of geographic structuring suggests recent expansion from a restricted distribution within that region. Beyond the Alps, two haplotypes predominate, accompanied by very similar variants; such star-shaped genealogies characterize recent population growth. Also, some rarer haplotypes have been independently introduced. Generally, haplotypes are well mixed here, often co-occurring at a locality. In North America and Australasia, some frequent haplotypes were likely directly introduced from Italy, because they were not found elsewhere in Europe. The rarity or absence in these continents of one or other of the two dominant European haplotypes suggests that import inspections have restricted repeated introductions. A skyline plot detects the recent demographic expansion but also indicates an earlier population decline in the native area. This may explain why the one-dimensional summary statistics F S and R 2 did not signal population growth. A review of 41 other studies that used DNA to analyse invasions of terrestrial molluscs documents considerable diversity in methodology. Studies using genetic data to date recent invasions probably should adjust standard substitution rates upwards. [ABSTRACT FROM AUTHOR]