An attempt was made to investigate the condition of infestation by the spionid polychaetes settled on the shell of the scallop Patinopecten yessoensis on the Okhotsk Sea coast of Hokkaido in Japan.The boring polychaetes were identified as Polydora variegata, Polydora ciliata and Polydora concharum. A large number of Polydora worms were observed to settle on the shell of the cultured scallop sowed in Abashiri Bay, and the dominant species was P. variegata throughout the year. It was also observed that the worms settled almost exclusively on the left valve of the scallop and the location of the burrows formed by them varied markedly with the species.In general, the larvae of P. variegata hatched out from the egg capsule mainly during the months of October and November and the settlement of these worms took place from January to April. On the other hand, the settling period of P. ciliata seemed to be found twice a year, namely, in July and from December to April. There was a tendency for the earliest settlement of both P. variegata and P. ciliata to occur on a narrow band around the periphery of the left valve. When numerous P. variegata settled on the shell of 1-year-old scallop in the first winter season after the start of sowing cultures, the place of attachment of the adductor muscle to the left valve overlapped the shell region heavily infested with the worms by the time of harvesting (2 years after sowing especially in Abashiri waters). In addition, the burrows often penetrate to the adductor muscle and hence the influence of P. variegata is of concern to our understanding of the low growth rate of scallops in Abashiri waters.