To clarify the variation pattern in reproductive allocation of Sagittaria trifolia L., 20 accessions collected from rice paddies located from Wakasa Bay to the southern extremity of Kii Peninsula were evaluated. These paddies were at altitudes of from 10m to 610m.A remarkable cline in plant biomass was found along the environmental gradient dependent on altitude and topographical features of the original place of accession. The plant biomass tended to increase with the decrease in altitude. Moreover, an accession collected from a small terraced paddy on a steep slope in a narrow valley showed extremely small value in plant biomass.Two kinds of propagules, achene and tuber, showed different variation patterns in reproductive allocation with relation to total biomass. Sexual reproductive allocation was little related to plant biomass, while a clear negative correlation was observed between allocation to tubers and total biomass.It was suggested that the greater amount of light supply in low altitudes results in larger plant size which leads to stronger competitive ability, by reducing the allocation to vegetative reproduction; a reduced amount of light supply and a less competitive situation with rice plants at higher altitudes, however, allows smaller plant size and greater allocation to tubers.Trade-off between number of tubers per plant and individual tuber weight suggested that this species has higher competitive ability at low altitudes, but higher reproductive capacity at higher altitudes.