Chara braunii is distributed worldwide and is the most common charalean species in Japan. This species is monoecious and produces numerous sets of sex organs, each of which consists of one antheridium and one oogonium, under laboratory culture conditions. In this study, we report that light intensity strongly affected the vegetative phase and sexual reproductive phase of this species. Under high-light conditions (70.0 μmol photons m−2 s−1), thalli grew but did not form reproductive organs. Under a low-light intensity (10.0 μmol photons m−2 s−1), algal bodies formed many reproductive organs. In addition, antheridia without the corresponding oogonia (lone antheridia) were observed under low-light conditions. The absence of oogonium primordia adjacent to the lone antheridium was confirmed by several microscopic approaches. The addition of liquid fertilizer increased the total number of sex organs and growth; however, the number of lone antheridia decreased with increasing fertilizer concentrations. Exogenously applied gibberellin did not affect the number of lone antheridia. These results suggest that regulatory mechanisms for the appropriate allocation of resources exist in this alga, similar to those reported in some land plants.