AbstractIn batch tests withChlorlla pyrenoidosa, Ankistrodesmus falcatus, Coelastrum cambricumandScenedesmus obliquusit is demonstrated that hydroxylamine as an extracellular product of metabolism is excreted by these algae into water. The hydroxylamine production is proportional to the number of cells and to the physiological state of the algae. A mixed population of aquatic bacteria does not show any hydroxylamine production, the hydroxylamine concentration in the medium of culture of algae being reduced by their presence.