One species of Hemichordata, Balanoglossus misakiensis, is then acorn worm originally reported from the intertidal zone of the Miura Peninsula on the Pacific Ocean side of Japan. We histologically examined the reproductive cycle of the population of this species, which inhabits only the sublittoral zone in the Sea of Japan. Testes and ovaries began to develop at the beginning of May 2003 and were almost mature in the latter half of June in males and in the first half of July in females in the same year. Subsequently, spermiation and spawning followed in the latter half of July in males and in the first half of August in females. Progress in maturation appeared to be related to increases in the water temperature. Although some experiments were conducted in aquariums to identify the conditions responsible for the synchronization of the occurrence of spontaneous spawning/spermiation, no clues were obtained. During the experiments, however, 11, 2, and 4 individuals out of the 67 used achieved spawning/spermiation on separate days. The occurrence of spawning/spermiation in the laboratory corresponded to the latter half of the switch from high tide to low tide on those days. Also in the field, it was known that they released the gametes according to this specific schedule. Therefore, it was suggested that, in the Japan Sea population of this species, the tide level may be a condition for synchronized spawning/spermiation.