Concern has been raised about the importation and transportation of GM B. napus, because feral herbicide-tolerant GM B. napus plants have been found growing around some major seaports in Japan. We should monitor the persistence of these feral B. napus populations including the herbicide-tolerant GM cultivars to prevent the negative impact of GM B. napus on native plant communities. In this study, we examined the plant numbers and the persistence of the B. napus at 19 sites around the Kashima seaport in Japan once a month from July 2004 to December 2005. In the results, the plant numbers ranged from 0 to 202 depending on the sites and/or the seasons. Out of 19 sites, we observed the seed dispersals of B. napus plants at only four sites. Of these four sites, we finally confirmed the self-sustainment of B. napus populations at two sites. Many plants growing at most of the sites disappeared before flower budding due to frequent human disturbances.