Nine species of Enteromorpha could be distinguished at the tidal flats and salt marshes in the southwestern part of the Netherlands up to now. The validity of some diagnostical characteristics is briefly discussed. A number of ecological notes is given. At the tidal flats, below the mean highwater mark, Enteromorpha species especially dominate on fine-grained sands, either attached to solid substrata (viz. shells) or basically interwoven with the sediments. At the salt marshes, above the mean high-water mark, the algal vegetation is much more variegated. Owing to their morphological plasticity, the taxonomical differences between the taxa are not always clear: especially E. prolifera appears to overlap E. torta on the one hand, and E. intestinalis on the other hand. E. prolifera is the most common Enteromorpha species on all levels of the flats and marshes, occurring in different morphological forms. Some notes on the seasonal aspects of the Enteromorpha vegetation are added. In spring all species begin to flourish. On the salt marsh Enteromorpha species develop before the phanerogams culminate. When the stands of halophytes become too dense, the Enteromorpha’s disappear rather quickly. At the flats and at open spots of the salt marshes the development of Enteromorpha continues till summer. Winter is the most unfavourable time, although most of the species can be found in small quantities.