ABSTRACT In contrast to most denitrifiers studied so far, Pseudomonas stutzeri TR2 produces low levels of nitrous oxide (N ₂ O) even under aerobic conditions. We compared the denitrification activity of strain TR2 with those of various denitrifiers in an artificial medium that was derived from piggery wastewater. Strain TR2 exhibited strong denitrification activity and produced little N ₂ O under all conditions tested. Its growth rate under denitrifying conditions was near comparable to that under aerobic conditions, showing a sharp contrast to the lower growth rates of other denitrifiers under denitrifying conditions. Strain TR2 was tolerant to toxic nitrite, even utilizing it as a good denitrification substrate. When both nitrite and N ₂ O were present, strain TR2 reduced N ₂ O in preference to nitrite as the denitrification substrate. This bacterial strain was readily able to adapt to denitrifying conditions by expressing the denitrification genes for cytochrome cd ₁ nitrite reductase (NiR) ( nirS ) and nitrous oxide reductase (NoS) ( nosZ ). Interestingly, nosZ was constitutively expressed even under nondenitrifying, aerobic conditions, consistent with our finding that strain TR2 preferred N ₂ O to nitrite. These properties of strain TR2 concerning denitrification are in sharp contrast to those of well-characterized denitrifiers. These results demonstrate that some bacterial species, such as strain TR2, have adopted a strategy for survival by preferring denitrification to oxygen respiration. The bacterium was also shown to contain the potential to reduce N ₂ O emissions when applied to sewage disposal fields.