Arion Ferussac, 1819, is a large, predominantly western Palearctic genus of slugs with a number of species that are of considerable agricultural and forestry concern. Many Arion species have been introduced throughout the world through commerce, and have established themselves as pests in predo- minantly synanthropic and other disturbed environments in the USA and Canada (Robinson, 1999). Since 1999, nation- wide surveys under the umbrella of the USDA North American Slug Project have been conducted to document the current distributions of non-native slug species. Arion species are as yet found to be absent from Puerto Rico, and surveys in Alaska and the Hawaiian Islands are incomplete. In view of the potential threat of new introductions of non- established Arion species, all specimens intercepted from incom- ing shipments from abroad must be identified by quarantine authorities in order to determine whether or not the associated commodity can be allowed to enter the country. Arion slugs are frequently intercepted at the US ports on commercial imports from Europe, particularly associated with shipments of mush- rooms, cut flowers, flower bulbs, horticultural plants and veg- etables. However, most intercepted slugs are immature, lacking the diagnostic genitalia that would facilitate their identifi- cation. Without internal examination, several species are difficult to identify, as the plethora of synonyms in the malaco- logical literature would suggest. The adults of some species resemble the immatures of others, and the external appearance of a species may also differ according to its geographic origin.