Spartina anglica is a classical example of recent alloploid speciation. It arose during the end of the nineteenth century in England by hybridization between the indigenous Spartina maritima and the introduced East-American Spartina alterniflora. Duplication of the hybrid genome (Spartina x townsendii) gave rise to a vigorous allopolyploid involved in natural and artificial invasions on different continents. Spartina anglica was first recorded in France in 1906, and since then, it has spread all along the western French coast. Earlier studies revealed that native British populations display consistent morphological plasticity and lack of isozyme variation. In this paper, we use different molecular markers (randomly amplified polymorphic DNA, intersimple sequence repeats and restriction patterns from nuclear and chloroplast DNA sequences) to analyse the genetic patterns of the French populations of S. anglica. Our results show that French populations are mainly composed of one "major" multilocus genotype. This genotype is identical to the first-generation hybrid S. x townsendii from England. Losses of few markers from this genotype are observed but are restricted to a few populations from Brittany; it is likely that they appeared independently, subsequent to their introduction. In southern Brittany, no hybrids between S. anglica and S. maritima have been found where the two species co-occur. All French populations of S. anglica display the same chloroplast DNA sequences as S. alterniflora, the maternal genome donor. These findings are consistent with a severe genetic bottleneck at the time of the species formation, as a consequence of a unique origin of the species. Both parental nuclear sequences are present in the allopolyploid populations, revealing that for the markers investigated, no extensive changes have occurred in this young species.