A comparison ofK M values for nitrate and nitrite reductases with uptake capacities of 6 marine diatoms indicated that intracellular accumulation of NO 3 − and NO 2 − is necessary for efficient functioning of the enzymes.Amphora coffeaeformis, Thalassiosira weissflogii andSynedra tabulata accumulated very little NO 3 − whereasTriceratium dubium, Achnanthes hauckiana andFragilaria pinnata accumulated 0·02 to 1· 5mM NO 2 − . Studies with inhibitors of nitrate and nitrite reductase indicated that nitrite reductase could be located i in a different compartment such as chromatophores as in higher plants. NH 4 + was also accumulated by the diatoms. Studies on the kinetics of glutamine synthetase and glutamate dehydrogenase indicated that the primary enzyme involved in NH 4 + assimilation could be glutamate dehydrogenase, because of its lowK 4 + and relatively greater activity than glutamine synthetase. Intracellular analysis of NH 4 + revealed that its concentration was well below theK 4 + for glutamine synthetase.