This paper deals with the occurrence of Potamogeton praelongus in the Netherlands, which has always been one of the rarer Potamogeton species in our country. In the 19th century and at the beginning of the 20th century it was recorded from several localities in the western and central part of the country, but nowadays it is almost restricted to the north-eastern part, its extinction elsewhere presumably being due to salination and pollution of the water. It is supposed that P. praelongus prefers percolation areas; it occurs chiefly on the border zones of sand and peat districts. Sometimes it is found on river clay. The species is often accompanied by Potamogeton lucens, Nuphar lutea, Hydrocharis morsus-ranae, and Sagittaria sagittifolia. If P. praelongus is compared with P. lucens, P. perfoliatus and P. pectinatus, all of them large, submerged aquatics, it turns out that they show a decreasing salt tolerance in the order P. pectinatus – P. praelongus P. perfoliatus – P. lucens – In the same order they are increasingly restricted to deep, clear water. A survey is given of the localities of P. praelongus in the Netherlands. Distribution maps are given of it and of the hybrid P. X decipiens, which is supposed to be P. lucens X P. praelongus.