The process of gonadal sex differentiation was examined histologically in Ayu Plecoglossus altivelis altivelis. Asymmetric paired gonads, each containing a few germ cells, were situated on both sides of a supporting mesentery 60 days after hatching (DAH). At 80 DAH, the gonads of fish smaller than 30 mm in total length had differentiated into two types, based on the number of germ cells; the gonad with many germ cells apparently into an ovary, and that with a few germ cells and a tube-like structure in the interstitial tissue, into a testis. In the former, meiosis began just after ovarian differentiation and the development of ovigerous lamella (by 100 DAH). On the other hand, a sperm duct formed in the interstitial tissue in the testis by 111 DAH. However, the testis contained few germ cells until 156 DAH. It is suggested that gonadal sex differentiation in Ayu occurs from 80 DAH, i.e., between 30 mm and 35 mm in total length. It is further suggested that gonadal development between 80 and 156 DAH is directly related to total length, rather than days after hatching.
