A study was made of the reproduction of Polydora variegata Imajima & Sato (Polychaeta:Spionidae) collected in Abashiri Bay on the Okhotsk Sea coast of Hokkaido, Japan, from May to December 1986. Large numbers of these bore into the shells of the cultured scallop Patinopecten yessoensis sown in Abashiri Bay. The spawning period, frequency of brooding females, number of batches per spawning period and number of eggs per batch of P. variegata are described. Although the life span of other Polydora species (hereafter Polydora) is generally thought to be ca 1-yr or less, the life span of P. variegata was found to be 2.5-yr after settlement on the shell. The main spawning period of both 0- and 1-yr-olds was from August to October when the sea temperature was relatively high. Some large worms, among the 0-yr-olds, laid their eggs as a string of capsules with 304 to 981 eggs per batch and on just one occasion. On the other hand, 1-yr-olds repeatedly produced egg capsules with 833 to 5715 eggs per batch. The principal reproductive population of P. variegata in Abashiri Bay seems to be composed of 1-yr-old worms.