Six species of spionid polychaete worms were identified from the shells of Pacific oysters,Crassostrea gigas(Thunberg), grown in Admiralty Bay, Marlborough Sounds, New Zealand:Polydora websteri, P. hoplura, Boccardia(Paraboccardia)knoxi, B. (P.)acus, B. atokouica, andB. chilensis. Previously unrecorded as a shellfish pest, the dominant speciesB. knoxiinduced the formation of shell blisters in subtidal oyster cultivations. Shell blisters contained little or no accumulated sediment but released hydrogen sulphide when punctured. The highest incidence of shell blisters was 57% and the lowest yield of oysters suitable for the half‐shell trade was 78%. The use of phenol as a vermifuge proved a more efficient method for extractingB. knoxifrom oysters than dissecting worms from shell blisters.