A microbiological study of the water and substrate in the Belon River was carried out during the summer of 1967, when the stocks of cultured oysters suffered very high mortalities. At the same time as the oyster mortalities there were dramatic decreases in the numbers of organisms, such as the diatoms and dinoflagellates, in the water. The numbers of bacteria, on the other hand, both in the water and in the surface layers of the substrate, reached very high levels, particularly amongst the anaerobic forms. It seems that the death of most of the organisms and the proliferation of the bacteria was due to the arrival of highly saline and cold water, which was rich in nutritive salts and poor in dissolved O sub(2). The augmentation of this water by organic material from various sources prepared the area for a bacterial explosion. In turn the bacteria helped to create an anaerobic situation both by their O sub(2) consumption and also by the production of hydrogen sulphide.