New observations, both in the marine environment and in the laboratory, indicate that the byssus in young post-larval bivalves has a hitherto unsuspected function - that of dispersal. The method of transport is analogous to the gossamer flight of young spiders. In the laboratory, small scale currents will induce the secretion of what seems to be a single long thread which, by increasing the viscous drag exerted on the young bivalves, enables them to be carried along on relatively small currents. The threads, which stain readily in Alcian blue (a stain selective for acid mucopolysaccharides) are typically long compared with the length scale of the animal itself: for instance an Abra 1 mm long can produce a 2-4-μm diameter thread of length greater than 3 cm. This mechanism, which we term byssus drifting, is functional from the late pediveliger stage up to at least 2.5 mm size in some species, and it has been found to be present in the 20 species tested so far and listed in Table 1 (a functional byssus has not been reported previously1 in post-larval bivalves of those genera marked with an asterisk). © 1976 Nature Publishing Group.