The benthic algal vegetation of the Gat van Ouwerkerk, a brackish inland water-basin, with a relative high and only slightly varying salinity, situated in the south-western part of the Netherlands, was surveyed in 1966 and 1967. Some ecological factors are mentioned. A survey of the vegetation-units is given. Four zones could be distinguished, the most striking belt being the permanently submerged vegetation where remarkable algae such as Dasya pedicellata (C. Ag.) C. Ag. and Bryopsis hypnoides Lamour. were found. Owing to the collaboration of divers some profiles could be drawn in which the vertical distribution and some seasonal aspects were emphasized. A comparison of the analysis of the sample-plots of Den Hartog (1964) with the author’s observations showed that the vegetation has strongly changed since 1961. The diagnosis of an unknown Acrochaetium species, living epiphytically on Dasya pedicelata, is given.