Production of byssus threads by the mussel Mytilus edulis is currently used for assaying natural substances capable of inhibiting attachment of marine invertebrates to immersed surfaces. The authors have adapted this method in order to obtain data on the effect of natural inhibitors on the different steps of mussel attachment: production of threads and choice of attachment site. From the results of a screening of purified extracts from several algal and marine invertebrate species from the North Atlantic coast, several potentially inhibiting or deterrent extracts have been selected.