Byssate bivalves can be attached to hard substrates by byssalthreads. Dissolution of the substrate to which they are attachedmay leave superficial bioerosion traces. This study reports thescars produced by byssus ofMytilus edulisandAulacomya atrarecorded in shells ofOstrea puelchana.Oyster valves were cut andgold-sputtered prior to scanning electron microscope observation.Each byssal thread leaves a characteristic scar on shells substrate.They etch an irregular trace on the substrate which may reacha diameter of several centimeters. These etching-traces compriseshallow round/oval holes of variable number and placement. Abundle of pits corresponding to fibers that compose the thread corewas identified in the interior of each hole. We suggest that this tracecould be included in the ethological class Fixichnia. The descriptionof this trace would allow adjusting the composition of fossilassemblages and therefore paleoenvironmental interpretations inthose cases in which mytilid shells have not been preserved. Therecognition of this trace along with other ones of tracemakerswith reotaxis or positive phototaxis could be useful in inferringthe life habit of fossil biogenic substrates and in reconstructingtheir taphonomic history.