The occurrence of epitokes of Platynereis bicanaliculata was examined in an eelgrass bed in Koajiro (Bay, Miura Peninsula, central Japan, using a fish lamp to establish a convenient and reliable sampling: design for environmental studies. The epitokes occurred in the period from late February to mid-November. The peak of epitokal swarming activity was in warm season, but its time in the season differed among years, i.e. late June in 1992 and early April in 1993, presumably, depending on weather conditions. The synchronous epitokal swarming was evidenced, superficially as asymmetrically semi-lunar with the major concentration of activity around a few days after full moon. The difference in the periodicity from the Californian forms exhibiting swarming activity around new moon is postulated to be attributable to populational genetic difference, if it is not due to the artificial induction of swarming by using a fish lamp in the present form. In either case, an annual relative density of the present species which may be an useful indicator of the environmental quality of coastal waters including eelgrass beds can be given by the samplings of epitokes using a fish lamp in the evening a few days after full moon and new moon in the warmer months from March to September.