Asexual worms of fissiparous strain of the planarian Dugesia ryukyuensis switch from asexual to sexual reproduction, if they are fed with sexually mature worms of Bdellocephala brunnea. This suggests that the sexually mature worms have a sexualizing substance(s) that induces the sexuality in the asexual worms. Here, we found by analysis of the sexualization that the cessation of the fission, namely their asexual reproduction, occurs immediately after the acquisition of sexuality. This result suggests that the downstream mechanisms induced by the putative sexualizing substance in B. brunnea become responsible for the cessation of fission. We also found that the decapitation triggers fission in the worms even after the acquisition of sexuality if they are not sexually mature, while the fully sexualized worms never fission even though they are decapitated. This result suggests that the cessation of fission takes place via at least two steps: (1) the mechanisms associated with the cephalic system; (2) other mechanisms independent of cephalic control.