Abstract An extensive survey of isozyme phenotypes in British populations of the amphidiploid salt marsh grass Spartina anglica and its putative parents has confirmed that the species arose by chromosome doubling in S. × townsendii, a sterile hybrid between S. maritima and S. alterniflora. Isozyme phenotypes and seed protein profiles indicate that S. anglica is almost totally lacking in genetic variation. Isozyme evidence also indicates that the parental species are characterized by low levels of genetic variation. The lack of variation in S. anglica is proposed as being due to a narrow genetic base resulting from a single origin, or a multiple origin from uniform parents; the fact that many populations are derived from very small founder populations; and because preferential pairing between identical homologous chromosomes prevents recombination between the divergent component genomes of the species. The low levels of isozyme variation that occur appear to be due to chromosome loss. The consequences for the future evolution of S. anglica, given its lack of genetic variation, are discussed.