The effect of spionid worm, Polydora websteri, infestation on the strength of shell valves of the sea scallop, Placopecten magellanicus, was investigated in relation to valve weight and degree of bioerosion. Compression tests indicated that the strength of upper valves increased with weight but were weakened in proportion to the degree of P. websteri induced bioerosion. Strength of the lower valves tested increased with weight and appeared largely independent of bioerosion. Scallop shells weakened by P. websteri boring are more vulnerable to crushing by predatory decapods