In the late summer of 1969 an investigation was made on water plants and vegetations of these in the peat holes of De Haak, part of the Nieuwkoopse Plassen (province S. Holland, the Netherlands). The pools are situated in marshwood peat and mostly have a thick layer of sapropelium. The water is about 1 m deep, is slightly oligohaline, has a medium calcium content, and is as yet hardly polluted. In the south the peat holes communicate with a ditch containing water which is comparatively rich in phosphate and nitrite. The increase in pollution and in the phosphate content to the south is demonstrated by the growing abundance of Ceratophyllum demersum and/or Vaucheria cf. dichotoma Ag., accompanied by species such as Elodea nuttallii and Lemna gibba. In the undisturbed parts of most peat holes there are luxuriant vegetations of Najas marina and several Characeae: Nitellopsis obtusa (Desv.) J. Groves, Nitella flexilis Ag., Chara hispida L., C. globularis Thuill. and C. aspera Willd. The first of these, Nitellopsis obtusa, formed abundant antheridia and oogonia, and in one place plants bearing ripe oospores were found. Oospores of Nitellopsis obtusa have been observed only a very few times in the world. Some ecological remarks are made, both on the species already mentioned and on other water plants occurring there, e.g. Fontinalis antipyretica, Nymphaea alba, Potamogeton pectinatus, Myriophyllum spicatum and Utricularia vulgaris. Further, the syntaxonomical position of the vegetations of Najas marina and Characeae is discussed. Finally the significance of De Haak is considered. De Haak represents the last remainder of the undisturbed ecosystem of partly dug out peat layers in the surroundings of Nieuwkoop. It forms an unique link in the series of peat holes areas (from mesohaline to fresh; from only peaty to peat lying on clay or sand) in the western part of the Netherlands. Unfortunately the vegetations in the water and on the banks are seriously threatened by the pollution of the water and by the consequences of the increasing recreation.