Little is known about the role of nitrate in evolution of bacterial energy-generating mechanisms. Denitrifying bacteria are commonly regarded to have evolved from nitrate-respiring bacteria. Some researchers regard denitrification to be the precursor of aerobic respiration; others feel the opposite is true. Currently recognized denitrifying bacteria such as Hyphomicrobium, Paracoccus, Pseudomonas and Thiobacillus form a very diverse group. However, inadequate testing procedures and uncertain taxonomic identification of many isolates may have overstated the number of genera with species capable of denitrification. Nitrate reductases are structurally similar among denitrifying bacteria, but distinct from the enzymes in other nitrate-reducing organisms. Denitryfying bacteria have one of two types of nitrite reductase, either a copper-containing enzyme or an enzyme containing a cytochrome cd moiety. Both types are distinct from other nitrate reductases. Organisms capable of dissimilatory nitrate reduction are widely distributed among eubacterial groups defined by 16S ribosomal RNA phylogeny. Indeed, nitrate reduction is an almost universal property of actinomycetes and enteric organisms. However, denitrification is restricted to genera within the purple photosynthetic group. Denitrification within the genus Pseudomonas is distributed in accordance with DNA and RNA homology complexes. Denitrifiers seem to have evolved from a common ancestor within the purple photosynthetic bacterial group, but not from a nitrate-reducing organism such as those found today. Although denitrification seems to have arisen at the same time as aerobic respiration, the evolutionary relationship between the two cannot be determined at this time.