To assess whether Mytilus edulis is selective in its secretion of byssus threads and entrapment of gastropods, experiments were conducted in laboratory aquaria and in the field. Exposure to dogwhelks (Nucella lapillus) or their effluent induced mussels to produce twice as many byssus threads as mussels exposed only to winkles (Littorina littorea) or those exposed only to seawater. There were no significant differences among treatments in the area over which byssus threads were secreted. Significantly more byssus threads were attached to the shells of dogwhelks than to winkles. Laboratory experiments produced broadly similar results to those in the field, but the level of response in the laboratory was greater. It is concluded that byssus threads were attached selectively to dogwhelks and that they may serve as a defence against predation.