Age determination of Margaritifera laevis was carried out in the Chubu-Nougu River, at Omachi in Nagano Prefecture, Japan. There were many juvenile mussels in the brook of Chubu-Nougu River. The relationship between age and shell length in juvenile mussels was investigated. In juvenile mussels, the annual rings on the shell surface roughly indicated the true age, but the rings of the ligament did not. Therefore, it is suggested that the growth rings of the ligament should not be used in age determination of juvenile mussels.