Résumé : With increasing frequency over the past 20 years, generally during the second half of June, Dutch and German beaches were covered with a massive layer, up to 2 m thick, of the alga Phaeocystis pouchetii. Increased anthropogenic inputs of nutrients, unbalanced in favour of nitrogen and phosphorus with respect to silica, promoted these conditions and reinforced the tendency of Phaeocystis to develop temporary, explosive blooms. These blooms caused a decrease in food resources for zooplankton, and increased deposition of organic material to the bottom, accumulation of organic material (dissolved or in the form of foam), and emission of volatile sulphur compounds. Means of control were unclear but it was possible that the food, cosmetic, textile and pharmaceutical industries could exploit the Phaeocystis mucus.