AbstractDuring microbial denitrification, NO is produced by reduction of nitrite by either the reduced high spin d1 hemes in a unique reductase (NIR) or at the expense of a blue copper protein that transfers electrons that move first to a type I copper and then to a type II copper in a unique trimeric NIR. This latter type of NIR is also produced by several denitrifying filamentous fungi. Reduction of NO is then carried out by either a specific cytochrome bc complex NOR in denitrifying bacteria or a unique cytochrome P‐450 in denitrifying filamentous fungi. NO is also produced by an anomalous reaction of a molybdoprotein, nitrate reductase (NAR), acting on an odd substrate, NO2−. NO is also reduced by a multiheme NIR that serves physiologically for reduction of NO2− to NH3. This type NIR reduces NO to either N2O, if only partially reduced, or NH3, if fully reduced, when it encounters NO. This multiheme NIR is very sensitive to cyanide. Transcription of the genes for NIR and NOR production in a denitrifier is activated by NO, a process that also requires the presence of the gene product, a transcriptional activator, NnrR.