Many metazoans convert the reproductive modes presumably depending upon the environmental conditions and/or the phase of life cycle, but the mechanisms underlying the switching from asexual to sexual reproduction, and vice versa, remain unknown. We established an experimental system, using an integrative biology approach, to analyze the mechanism in the planarian, Dugesia ryukyuensis (Kobayashi et al., 1999). Worms of exclusively asexual clone (OH strain) of the species gradually develop ovaries, testes and other sexual organs, then copulate and eventually lay cocoons filled with fertilized eggs, if they are fed with sexually mature worms of Bdellocephala brunnea (an exclusively oviparous species). This suggests the existence of a sexualizing substance(s) in sexually mature worms. Random inbreeding of experimentally sexualized worms (acquired sexuals) produces an F1 population of spontaneous sexuals (innate sexuals) and asexuals in a ratio of approximately 2:1. All regenerants from various portions of innate sexuals become sexuals. In the case of acquired sexuals, head fragments without sexual organs regenerated into asexuals though regenerants from other portions became sexuals. Thus, we conclude that neoblasts, the totipotent stem cells in the planarians, of acquired sexuals remain “asexual” and the worms require external supply of a sexualizing substance for the differentiation of sexual organs and gametes. On the other hand, some, if not all, neoblasts in innate sexuals are somehow “sexual” and do not require external supply of a sexualizing substance for the eventual differentiation of themselves and/or other neoblasts into sexual organs and gametes. It is also shown that sexuality in acquired sexuals is maintained by the putative sexualizing substance(s) of their own. The sexualization is closely coupled with cessation of fission, and the worms seem to have an unknown way of controlling the karyotype. Our integrative approach integrates multiple fields of study, including classic breeding, regeneration, and genetics experiments, as well as karyotyping, and biochemical and molecular biological analyses; none of which would have revealed much about the intricate mechanisms that regulate sex and fission in these animals.