This paper deals with an analysis of the population of Spartina x townsendii s.l. occurring in the coastal area of the Southwest Netherlands. The analysis was carried out at the Delta Institute for Hydrobiological Research in Yerseke and the laboratory of Plant Taxonomy of the Agricultural University in Wageningen. Seventy-six plant samples were taken from twenty locations, ranging from tidal to non-tidal, and from marine to brackish habitats. Fifteen morphological characters were measured, whereas plant community descriptions were made and soil samples taken. The somatic chromosome number was assessed from thirteen samples. A principal components analysis was carried out of the main morphological characters. It appears that, morphologically, a townsendii-type and an anglica-type can be distinguished, with considerable variation occurring in the anglica-lype which is probably partly ecotypical. However, under various ecological conditions plants, which originate from anglica-type i populations assume a dwarf form, which cannot be distinguished from plants of the townsendii-type. It was also found that sometimes townsendii-lype plants occur with a small amount of normal, fertile pollen, and even plants with some normally developed fruits. Cytological investigations show that plants, which should be classified as S. anglica on morphological grounds (and also the dwarfed plants, originating from anglica-type plants), can be found with either the diploid or the tetraploid chromosome number. These results conflict with those of Marchant (1967,1968), who concluded that S. anglica is tetraploid and that diploid plants, classified as S. x townsendii s.str., are always sterile. This means that a possible separation on the basis of morphological characters is not corroborated by a separation based on chromosome numbers. Therefore, without further research, the morphological types cannot be classified as species.