In 1962 Dr. C. den Hartog (Leiden) has found the green alga Capsosiphon fulvescens (C. Ag.) Setch. & Gardn. for the first time in the Netherlands, viz. at the Beningerwaard. In 1965 and 1966 we collected Capsosiphon at six different localities in the south-western part of the Netherlands (fig. 2). Taxonomy and geographical distribution are shortly dealt with. Moreover the ecology of the species is discussed. Capsosiphon has been found in oligo-, meso- and polyhaline waters; it is an euryhaline species. It occurs in the upper part of the eulittoral region or, in localities where no tidal movements are present, around or above the water level. The alga shows a preference for eutrophic, rather strongly polluted water. At some places the favourable influence of rain water running off along the slopes, should be taken into account. The vegetation units in which the species has been observed are the following algal communities: Blidingietum minimae, the adjacent Bangieto-Urosporetum, and, separately, the Enteromorphetum prolifero-intestinalis. Capsosiphon is surely to be found in other localities in the Deltaic area, not only because it has been often overlooked but also because one is creating more and more suitable biotops for this alga.