A clear modification of sex expression in Sagittaria latifolia from male or trimonoecious to female was induced by a foliar spray of gibberellic acid (GA) at concentrations of 10, 100, and 1000 ppm. This modification began two weeks after the initiation of treatment. The effect was greater at a higher GA concentration. The number of racemes was increased by the treatment. On the other hand, male and perfect flowers were produced in the female line by a foliar spray of paclobutrazol (PBZ) at concentrations of 100 and 1000 ppm two weeks after the initiation of treatment, and a greater number of male flowers was produced in the trimonoecious line by the spray. The number of racemes was decreased by the PBZ application. GA was effective for the internode elongation of rachises and scapes; on the contrary, PBZ was effective in shortening the internodes. Therefore, it was clarified that endogenous GA controlled sex expression, raceme production, and internode length in this species. Female flowers in the male line developed into fruits with a normal seed; further, female flowers that had been pollinated with pollen grains from male flowers of the female line also developed into fruits with a normal seed. These results will be useful for the breeding of S. latifolia.