Overwintering habits of the land snail Vertigo angustior (Mollusca) During a mollusc survey in 2004 en 2005 in the province of Zuid-Holland special attention was paid to Vertigo angustior. The species was observed overwintering in large groups, up to hundreds of specimens together, between mosses (especially Brachythecium rutabulum (Hedw.) Schimp.) in small patches of poplar forest (Populus nigra and Populus alba) in the old dunes. As far as we know, this gregarious behaviour is described here for the first time. Many of the snails were found in a position where they had incorporated a moss leaf into the epiphragm closing the shell’s aperture, thereby achoring themselves to the vegetation, possibly for stability.