Hatatatenumeri dragonets (Repomucenus valenciennei) in Tokyo Bay, Japan, were collected by beam trawl (mesh size 1.6 mm-bar measure) from 15 stations every two months from September 1990 to September 1991, and similarly trawl (mesh size 3 cm-stretched measure) from 20 stations every season from June 1990 to February 1993. Settled juvenile and subadult fish were caught by the 1.6 mm mesh trawl, whereas subadult and adult fish were caught by the 3 cm mesh trawl. Dragonet distribution was estimated from abundance (indiv per tow) at each sampling station for each year class, the abundance data being divided into year classes from length frequency data. Settled juveniles appeared in autumn throughout the bay, with abundance being greater in the northern (innermost) part. The dragonet was restricted to the southern (close to the mouth) part in summer, but subsequently spread throughout the bay, with abundance being greater in the northern part from autumn to winter. Because of this seasonal change in distribution, the dragonet was believed to migrate seasonally between the northern and southern parts of Tokyo Bay, hypoxia in summer possibly causing the southward migration of immature fish from the northern part, in addition to restricting the distribution of mature fish at that time.