A system for the biological removal of NOx from fuel flue gas was investigated using the unicellular microalga Dunaliella tertiolecta. When nitric oxide (NO), the main component of NOx in hue gas, was supplied to the algal culture in a bioreactor with a 2-m column in the light at concentrations ranging from 25 to 500 ppm, about 65% of the NO was removed. Under these conditions, cell growth was not affected by the concentration of the NO supplied, and about 1.6% O-2 was constantly evolved by photosynthesis. About 30% of the NO was removed by the medium without cells at 2% O-2, in which ease the NO was probably photochemically oxidized by Fe3+ present in the medium. In cell cultures without Fe3+, however, 65% NO removal was achieved. In the dark, on the other hand, the rate of NO removal was governed by the amount of O-2 supplied in the inlet gas, i.e., achievement of a NO removal rate similar to that achieved in the light required the presence of O-2 at 2% or more, and NO removal did not occur without the supply of O-2. It is thus clear that both algal cells and O-2 are essential in the reactor system. NO removal is assumed to proceed as follows : NO in the gas is first dissolved in the aqueous phase, after which it is oxidized and assimilated by the algal cells. The results of investigations under various culture conditions indicate that the dissolution of NO in the aqueous phase is the rate-limiting step in this reactor system.