Rather frequent occurrences of the abnormal individuals of the ayu, Plecoglossus altivelis Temminck et Schlegel, were recently experienced in nature and culture pond, and their abnormalities were more often localized at vertebral column.Then, it became necessary to trace the normal structure of the vertebral centra as our basic knowledge, on which teratological study must stand.<BR>Length and diameter of the centra of 400 specimens were measured by different growth stages and by sexes(Table 1).The anadromous ayu were collected in the River Yahagi, Aichi Prefecture.As regards lacustrine ayu, young fish collected from the Lake Biwa were cut off the adipose fin for marking, and released in the River Kadowasa, Gifu Prefecture.These fish were recaptured and used for this study.Hatchery reared ayu were obtained by artificial crossing of a pair of adult fish as brood stock taken from the River Kiso, Aichi Prefecture.<BR>In the young below 3 cm in standard length(SL), all the centra in column are near equal in length, but, along with further growth of fish, the centra located anteriorly in caudal vertebrae grow longer than others, and in the fish over 12 cm SL, centra located posteriorly in abdominal vertebrae also grow longer than those of other abdominal vertebrae.The diameters of centra in the fish 2 to 5 cm SL show no difference by its position throughout the column, but in larger fish, those located near transition from abdominal to caudal vertebrae grow larger.<BR>Sexual difference was found on adult male which has wider diameter of centra located near the transition region than that in female, and similar tendency was noted for the length of centra.<BR>The young ayu of anadromous form, 7-8 cm SL, have wider diameter of centra than that of lacustrine form of similar body size, and the difference becoming more evident between the males(14-16 cm SL)of the two forms.And, in the young ayu of 7-8 cm SL, the length of centra of hatchery reared fish was found larger than that of lacustrine form, but the diameter and length of centra of the former(4-5 cm SL)were found smaller than those of anadromous form.Differences in both length and diameter of centra were not observed between anadromous and hatchery reared ayu when they reached 11-12 cm SL..