The spread of plant populations into new habitats and their establishment there is an important object of ecological research, particularly for those plant species consisting of cultivated, weedy and wild forms. One of these species is Beta vulgaris L., which is taxonomically divided into the subspecies vulgaris (sugar beet, Swiss chard, red beet, some weedy beet forms) and maritima (sea beet). In northeastern Europe, the distribution range of the different Beta subspecies is not well known. The aim of this study was to investigate the origin of sea beet populations at the Baltic Sea coast of Germany by collecting seed samples and performing RAPD-PCR analysis. Theoretically, the spread and the establishment of beet populations in the wild could be the result of various different processes: (1) introduction of seeds from wild populations, (2) naturalization of cultivated or weedy types after introduction into wild habitats, and (3) a combination of (1) and (2) with subsequent hybridisation and evolution into an intermediate genotype. Genetic analysis suggests that the Baltic Sea coast populations are pure wild beets (1) and not weedy or escaped sugar beets (2 or 3). In a UPGMA phenogram, the wild and the cultivated beets could be clearly distinguished from one another. In addition to the strong evidence that German sea beet originated from Danish sea beet, the possibility of seed dispersal between these groups via seawater was demonstrated. Beta vulgaris seeds survived a long period in seawater and up to 2% of the seeds still germinated after 28 weeks. The fact that sea beet populations can now be found sympatrically to flowering sugar beets and weed beets in coastal agricultural areas offers new possibilities of gene flow. This is important fundamental knowledge for the risk assessment of gene flow from transgenic sugar beet.