Abstract:The relationships between sex expression, pollinators response and pollen flow in a field population of Sagittaria trifolia L. were examined. These plants were monoecious with the number of male flowers 3.6 times of female flowers. The anthesis of two sexes was only one day, but the floral display was different. The female flowers opened firstly and bloomed more flowers in one day in a raceme; however, the male flowers opened few flowers in one day and lasted longer time in bloom. The floral display thus appeared large fluctuation at the daily ratio of two sexual flowers in the population. In 2 612 pollination activities, the pollinators tended to visit female flowers. The mean number of visits received per female flower was 1.4 fold than that of per male flower. To test effect of the different floral display on pollen flow, the authors estimated pollen dispersion by labeling the anthers of male flowers with safranine dye in two experiments. When the male flowers were more abundant, most dyed pollen dispersed close to the labeled plant. As the female flowers having relatively high proportion in the population, the labeled pollen grains moved in far distance.