We report that an eelgrass bed Zostera marina L. at the Aland Islands, northern Baltic Sea, is dominated by a. single genotype which extends over an area of approximately 160 x 40 m. In total, 47 individuals were sampled and genotyped for 6 highly polymorphic microsatellite loci. Forty-four of the ramets revealed the same 6-locus genotype, including 3 identical heterozygous loci. They were thus assigned to the same genet (= clone). To our knowledge, it represents the largest marine plant identified thus far. Based on estimates of horizontal rhizome growth rates, this clone may be more than 1000 yr old. The remarkable phenotypic plasiticity of a single genotype which dominates this site illustrates that there is no simple one-to-one relationship between genetic diversity and population persistence in changing and stressful environments.