An assay system has been established for the sexual induction in the OH strain, an exclusively fissiparous (asexual) strain, of Dugesia ryukyuensis by feeding them with sexually matured worms of Bdellocephala brunnea, an exclusively oviparous (sexual) species. In this assay system, asexual worms gradually differentiated sexual organs, namely the ovary, testis, genital pore and yolk gland in this order, and eventually mated and laid cocoons filled with fertilized eggs. Although the OH strain worms were believed not to have any sexual organs, a pair of undeveloped ovaries with a few oogonia were detected by an intensive histological search. Along with the progression of sexualization, five distinct stages were histologically recognized: In the first stage, the ovaries became larger enough to be externally apparent; oocytes appeared first at stage 2; the primordial testes emerged at stage 3; a genital pore opened, yolk gland primordia developed and spermatocytes appeared at stage 4; and finally at stage 5 matured spermatozoa and yolk glands were formed. Worms in stages 1 and 2 but not in later stages returned asexual if feeding on B. brunnea was interrupted. Furthermore, when the worms at stage 3 onwards were cut posterior to the ovaries, all the tail regenerants developed eventually into fully sexualized worms. Taking these results in account, we have concluded that the process of sexualization has a point-of-no-return between stages 2 and 3. It is likely also that the testes, even the primordia, play an important role in the maintenance and development of sexuality.