In the amphidromous form of ayu Plecoglossus altivelis Temminck et Schlegel, generally the prelarvae hatched in rivers in autumn are drifted down into the sea, where they live throughout the larval and early juvenile stages until spring. In the Shimanto estuary, however, an appreciable number of postlarval ayu occurred not only during March to May, but also during October to February. Furthermore, there was little difference in the size and age between the larvae occurring in the estuary and sea. In the estuary, they reside in the bank waters, during 10-75 day ages and 10-30mm SL. Thereafter, they seem to ascend the river without going out to the sea. The growth rate estimated from the otolith daily rings was higher in the estuary than in the sea beach.