Arion fuscus, also known as the "dusky arion", is a species of small air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae, the roundback slugs.

Arion subfuscus (Draparnaud, 1805) is considered by some authors to be a partial synonym for Arion fuscus. Arion subfuscus (Draparnaud, 1805) and A. fuscus (Müller, 1774) have very similar morphology and have overlapping distributional ranges in northwestern Europe. Arion fuscus is widespread throughout Central, North and East Europe whereas A. subfuscus is restricted to West Europe. (For practical purposes the two are synonyms in Germany and in the Czech Republic.) The two taxa are referred to as the species complex Arion subfuscus/fuscus both members having several evolutionary lineages. They are only separable by alloenzyme analysis and gonad type.

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