In certain areas of the Baltic proper strong blooms of potentially toxic bluegreen algae (Nodularia) and dinoflagellates were observered. The observations were supported by sharp fluorescence maxima in the water. The extent of oxygen depleted, sulphidic, bottom water in the Gotland Basin had increased markedly since the investigation previous month, as had the sulphide concentrations in the same water. The lowest observed oxygen concentrations in the Kattegatt (50% saturation) were found in the southern part of that area and in the Sound. No actual oxygen depletion was observed. The surface waters in the whole investigated area were depleted in nitrogen, while both phosphate and silicate were readily available.