The variability of the principal differentiating characters of Spergularia marginata and S. salina was studied by means of analyses of population samples. Both species proved to be sharply definable in the Netherlands; no intermediates were found. In certain habitats a form of S. marginata occurs with exclusively unwinged seeds. It seems that S. salina, on the other hand, has a form with broadly winged seeds only. So far it was found in but a single Dutch station.