Anabaena cylindrica was immobilized in calcium alginate beads and was placed in a batch reactor in the presence of a glutamine synthetase inhibitor (methionine sulfoximide). Ammonia was released in the medium during two days with a rate of 0.35 μ moles h−1mgChl−1. Addition of nitrite to the medium increased the ammonia production as cells used the nitrite reductase pathway to form ammonia. When reactors were placed in anaerobiosis by N2 bubbling, ammonia production was sustained several days and the total ammonia formed was about two fold higher than in aerobiosis. Long term effects of MSX, nitrite and anaerobiosis are discussed.