We quantified the mollusc assemblage composition in eight riverine forests inhabited by the door snail Alinda biplicata (Montagu) in Central Poland where it occurs at the north-eastern border of its distribution range. In each location, eight random 0.25 m2 plots were quantitatively sampled from a 400 m2 core area that was searched for additional species. A total of 54 species were found, composed of 46 terrestrial snails and slugs, six freshwater gastropod and two clam species. Abundances ranged from 220 to 4,400 ind.m–2 per location, with a maximum of 2,200 individuals per plot. The number of taxa ranged from 17 to 34 per location and from 3 to 23 per plot. A. biplicata occurred in each randomly sampled plot. The highest number of co-occurrences with Alinda was found for Carychium tridentatum and Nesovitrea hammonis. Additionally, forest-specific, wetland-specific and even dry habitat-specific snails can use the same patch of microhabitat. The lack of narrow ecological specialisation in A. biplicata may favour its presence in man-made habitats where it is sometimes considered an invasive native gastropod. The species list included invasive Arion vulgaris and some other species with a mainly West European distribution, such as Oxychilus draparnaudi, O. alliarius, Arion rufus, Cepaea nemoralis and C. hortensis. The eastern borders of the native ranges of these species reach into the western part of Poland but in the sampled region they seem to be recent arrivals. Our results indicate that “western” (Atlantic) faunal elements benefit from more balanced microclimate in wet floodplain forests, which possibly reduces macroclimatic constraints at their distribution border.