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Recent studies have suggested that reproductive interference, a deleterious interspecific interaction in the mating process, plays an important role in biological invasions. In the system of plant species, however, the border with the pollen limitation has often been vague in past studies. This study, using field and laboratory experiments and field observations, examined the reproductive success of an endangered native herbaceous plant, Veronica polita lilacina, in the context of the reproductive interference by the alien congener, V. persica. The auto-pollination experiment confirmed that both species can usually produce seeds even without external conspecific pollens. Results of the artificial pollination experiment demonstrated that pollination with the heterospecific pollens significantly decreases the number of seeds in the native species, but not in the alien species. A transplant experiment revealed that the coexistence with the alien species reduced the fruiting success of the native species. Field observations have shown the interaction between two species in the native patch with only one intruding alien species. They demonstrated that native individuals placed closer to the alien individual suffered a greater decrease in fruiting success and the seed production and that the alien intruder produced no seed. These results demonstrate that species that could reproduce via the auto-pollination suffered the reproductive interference and that the native species also exert the resistive reproductive interference slightly. These interactions can explain the displacement pattern of the native species by the alien congener in Japan.