Japanese sea bass Lateolabrax japonicus is a common euryhaline marine fish distributed in eastern Asian coastal waters, of which life history can be characterized by occasional or seasonal entry to freshwater habitats. Otolith microanalysis for Sr:Ca ratio by a wave-length dispersive electron microprobe was utilized to determine life history trajectory of the Japanese sea bass distributed in Ariake Sea. Laboratory-raised sea bass juveniles which were maintained at various constant salinity regimes and serially decreasing-increasing salinity conditions showed that Sr:Ca ratio was significantly lower in fishes exposed to freshwater than in those to brackish and sea water over salinity 10 ppt. Sr:Ca ratio chronology of a yearling collected in the Chikugo River estuary of Ariake Sea located in western Japan, revealed that fish had immigrated to freshwater habitat during the early life stage and moved between freshwater and estuarine areas. An adult sea bass captured around a spawning ground was confirmed to migrate seasonally into freshwater area by Sr:Ca ratio chronology. These findings validated that otolith microanalysis is a useful tool to determine life history trajectory for Japanese sea bass from very early Life through adult stages.