A strong population of the terrestrial slug Arion ater s. l. is reported from the European and Asian parts of Istanbul, Turkey. This is the first confirmed report of this large, conspicuous taxon from Turkey and from Asia. Our samples from five synanthropic sites indicate that it is already well established. Partial sequences of the mitochondrial COI gene (cytochrome c oxidase subunit I) place the Turkish slugs in a small clade shared with a few specimens from western France, perhaps indicating the origin of the Istanbul population. The next closest haplotypes (9% difference) fall within the clade identified as Arion ater s. s. This fits with the genital morphology of the Turkish slugs, which is most similar to the ater-form of A. ater s. l. Our discovery also puts a new light on the recent report of the highly invasive pest slug Arion lusitanicus auct. non Mabille, 1868 (often called Arion vulgaris Moquin-Tandon, 1855) in Isparta, which was identified only on the basis of external morphology. As reliable morphological distinction of these two species requires examination of the genital anatomy, the specimen from Isparta should be reinvestigated.