The comparative investigation of the morphological, physiological and denitrifying properties of 8 freshly isolated strains of denitrifying bacteria from various taxonomical groups leads to the following conclusions: a) In agreement with earlier authors it was found that the reduction of nitrate as well as of nitrite is of strongly adaptive nature. b) The role of oxygen tension in the medium for the formation of the reductase systems is not uniform with all strains. In contrast to Pichinoty et al. we found, however, no organism which needed complete anaerobiosis for the adaptive formation of nitrate or nitrite reductases. c) Some of the nitrate adapted strains showed a faster rate of nitrate reduction than of the reduction of the produced nitrite. In consequence a transient accumulation of nitrite occurred when nitrate was to be reduced. d) Hydroxylamine or "artificial" proton donators such as Diphenyl-p-phenylendiamine or Phenylendiamine could be used by our strains to a different degree. It seems premature to draw far-reaching conclusions regarding the biochemical mechanisms of denitrification from experiments with such H-donators (comp. lwasaki, 5, 6). e) The rate of denitrification exerted by the various strains is rather differently affected by the O-tension in the medium. Denitrification by strain 21 for example was strongly inhibited by even very low oxygen-tensions. Some other organisms reduced nitrite at an oxygen tension of 10 to 15 mm Hg nearly as fast as under anaerobical conditions, were, however, strongly inhibited by tensions of 153 mm (air). On the other extreme denitrification by strain 19 was not at all affected by an oxygen tension of 153 mm (air) provided glucose was available as an H-donator.