SUMMARYData are presented on distribution and ecology ofVaucheria compactain the Netherlands. A description is given of a sociation ofV. compacta, an estuarine community, often showing mass occurrence. From culture experiments in salinity ranges it is concluded that there are no “salinity races” but that a non‐modificative morphological variation is present in some antheridial characters. As a consequence of this, a new variety is described:Vaucheria compactavar.dulcis. At some occasions monoecious filaments were observed within this dioecious sp