A survey is given of the occurrence of filamentous algae from dune pools situated in the calcium carbonate-rich dune area at Oostvoome and Egmond-Castricum as well as the calcium carbonatepoor Wadden area at Terschelling. Spirogyra and Oedogonium were important and often dominant components at these sites and were represented by at least 35 and 20 species respectively. Accompanying species belonged to Zygnema, Mougeotia, Bulbochaete, Microspora and Tribonema. In several pools Chara was dominant. Mass occurrence of filamentous algae was frequently observed. The seasonal dynamics of the components in floating mats were described. Periodicity phenomena of components and individual Spirogyra and Oedogonium species were studied. A peak in spore production of Spirogyra was situated in the second half of May and the first half of June. Based on physico-chemical data, the water of most of the investigated pools can be characterized as mesotrophic freshwater with a moderate or high amount of calcium and bicarbonate. Water quality differences between Terschelling (low alkalinity) and Egmond-Castricum/Oostvoome (high alkalinity) were reflected in their species composition. Most of the species from the dune pools are also known from other more nutrient-rich habitats in The Netherlands, but there are differences in quantities and combinations of species.