SUMMARY An attempt was made to detect interaction between photosynthesizing algae and denitrifying bacteria by cultivating Chlorella sp. with Pseudomonas stutd. Deni- trification was inhibited by illumination of the mixed culture after a dark period of 3 hr. It is suggested that immediate inhibition is due to the molecular oxygen pro- duced by the suspended algae and distributed homogeneously in the culture. A delay of this inhibition may be caused by uptake of oxygen by the denitrifiers at a more rapid rate than its production by the algae. Thus, the quantitative aspects of this interaction depend upon the particular conditions of cultivation used. The ecological implications are discussed. A situation of this type was tested in vitro by cultivation of algae with a deni- trifying bacterium which was found as a contaminant of algal cultures. The work was stimulated by the observation of vigorous gas production in liquid cultures of a ChZoreZZa sp. still contaminated by bacteria, which had been accidentally stored in the dark for several days. The gas was found to consist of CO,, 10-15 % (v/v) N,, and traces of 0, and N,O. Nitrate, initially present in the culture at 0.3% (w/v), disappeared in a period of about 18 days. The only hydrogen donor present and possibly used by the denitrifying micro-organisms must have been decomposing algal material. Addition of other hydrogen donors (e.g. peptone, yeast extract) vigorously accelerated the disappearance of nitrate. Shaking and aeration retarded, but did not prevent, the loss of nitrate and appearance of elementary nitrogen. The sensitivity of the denitrification to an accompanying algal photosynthesis was then studied. Since the discovery of denitrification, its occurrence in the presence of oxygen has been discussed in many papers which often reported contradictory results.