Coreopsis lanceolata has been invading native grassland on gravelly riverbeds in Japan, and is designated as an invasive alien species of Japan. An ecological study on C. lanceolata was conducted in the riverbed of Kiso River, central Japan. We surveyed species composition, flowering characteristics, seed reproduction and seed bank of C. lanceolata community. The height of C. lanceolata was taller than the indigenous plants, and dominated the upper layer of the gravelly riverbed grassland. The numbers of shoots with flowers were in the range from 15 to 25 / m2. The average of the seed number per head was 98 in the study site, which was more than twice as that of the native C. lanceolata communities in North America. These characteristics show that the C. lanceolata community on the study site is able to produce more seeds than the native communities. There were 460 - 865 seeds / 0.4m2 in the soil of the study site. It is important to give considerations on prevention of seed dispersion and germination, and of seed supply to soil in order to control C. lanceolata communities in Japan.
