Otolith microstructure of 1073 of larvae and juveniles ayu Plecoglossus altivelis was examined to establish the method to discriminate between land-locked fish (LAKE) and amphidromous fish (SEA). Their birth date and otolith growth rate during February were determined by ageing fish with daily increments of sagittal otolith. Based on the comparison of frequency distributions of birth date and otolith growth rate between LAKE and SEA, fish with the birth date before Sep. 27 and/or the otolith growth rate below 1.9μm/d were estimated as LAKE, while those with birth date later than Nov. 23 and/or the growth rate above 3.6 μm/d were SEA. Fifty-six percent of LAKE and 39% of SEA were fitted to the above indices, and the theoretical accuracy for judgement was 98%. Blind test of 50 specimens showed that 43% fish could be judged, and the accuracy was 84%.