Many workers have reported that the adult population of the small prosobranchPeringia(=Hydrobia) ulvaereaches its maximum density on muddy‐sandy shores in a broad belt at approximately mid‐tide level. Yet it has also been reported that some float for at least part of the tidal cycle and at low tide burrow in the sand.Experiments in the laboratory and observations on the shore suggest that under natural conditions the animals float on the surface film of the water by means of a mucoua raft, which also acts as a food net. After a time, which is geared to the tidal oscillation, they submerge at mid‐tide level. When the tide has receded, the animals crawl about on the surface of the mud then burrow in the sand for several hours, resurface, and launch themselves on the surface film of the water between the ripple marks. By this time the tide is approaching mid‐tide level again and the cycle of behaviour is completed.Thus these animals, formerly regarded as mainly benthonic, spend part of their time as pelagic plankton