The morphological, physiological and denitrifying properties of 8 freshly isolated strains of denitrifying bacteria from various taxonomic groups were investigated. The reduction of nitrate and nitrite is of strongly adaptive nature. The role of O tension in the medium for the formation of the reductase systems is not uniform with all strains. No organism requires complete anaerobiosis for the adaptive formation of nitrite reductases. Some of the nitrate-adapted strains showed a faster rate of nitrate reduction than of the reduction of the produced nitrite; a transient accumulation of nitrite occurred when nitrite was to be reduced. Hydroxylamine of "artificial" proton dontors such as diphenyl-p-phenylendiamine of phenylendiamine were used to a different degree. The rate of denitirfication by the various strains is differently affected by the O2-tension in the medium. Denitrification by strain-21 was strongly inhibited by even very low O2-tensions. Some other organisms reduced nitrite at an O2 tension of 10 to 15 mm Hg nearly as fast as under anaerobic conditions. They were strongly inhibited by tensions of 153 mm (air). Denitrification by strain-19 was not affected by an O2 tension of 153 mm (air) provided glucose was available as an H-donor.