The biological process of dinitrogen (N2) gas formation from fixed nitrogen compounds such as nitrate (NO3−) plays an important role in maintaining homeostasis of the global environment. Bacterial denitrification was long considered the sole biological reaction responsible for this condition, and its systems have been characterized in detail (3, 8, 20). Denitrification physiologically functions as anaerobic respiration in which NO3− is used as the terminal electron acceptor when oxygen (O2) is unavailable. Known bacterial denitrifying systems consist of four steps that successively reduce NO3− to N2 and involve nitrite (NO2−), nitric oxide (NO), and nitrous oxide (N2O) as intermediates. Production of the enzymes catalyzing each step is induced by nitrogen oxides and suppressed by O2 (3, 20). During the past decade, denitrifiers have been discovered in a variety of taxa including filamentous fungi (9, 10), yeasts (15), and actinomycetes (1, 11). All of these novel denitrifiers produce N2O as the major denitrification product. Both nitrogen atoms in the N2O product are derived from nitrate (or nitrite) (1, 9-11, 15). The phenomenon is therefore defined as denitrification, since the process should include the formation of an N—N bond (20). This system in several fungi is localized at respiring mitochondria, where it acts as anaerobic respiration, as it does in bacterial systems (5, 13, 16). Another unique feature of the fungal system is the involvement of cytochrome P450 (P450nor) as NO reductase (Nor) (6). Biological processes other than bacterial denitrification evolve N2 from fixed nitrogen compounds, but their molecular mechanisms and physiological significance remain to be elucidated. We identified simultaneous fungal codenitrification and denitrification, in which a hybrid N2 species is formed by combining two nitrogen atoms derived from NO2− and from other nitrogen compounds (10, 14). The denitrifying fungi Fusarium solani and Cylindrocarpon tonkinense evolve hybrid N2 species (10). The fact that the denitrifying fungus Fusarium oxysporum evolves N2O instead of N2 by codenitrification only when a nitrogen compound in addition to NO2− such as azide, salicylhydroxamic acid, or ammonium (NH4+) is available (14) suggests that the mechanisms of this process differ among fungal species. Anammox is a third N2-generating metabolic process that has been identified in the strictly anaerobic chemolithotrophic Planctomycetales (12), in which NH4+ is combined with NO2− to form N2. The actinomycetes form a unique taxon among gram-positive bacteria. Although actinomycetes naturally proliferate in soil and in aqueous environments, little is known about how they accomplish denitrification. Denitrifiers also occur among actinomycetes (1, 11), and the system of Streptomyces thioluteus has been characterized previously (11). All of the actinomycete denitrifiers found in these studies evolve N2O from NO3− or NO2−, and thus, no known actinomycete strains contain a complete denitrifying system that can thoroughly reduce NO3− to N2. The present study continues screening for denitrifying actinomycetes (11) by using a highly sensitive N2 detection system equipped with an isotope mass spectrometer and NO3− labeled with a stable isotope ([15N]NO3−). The results showed that Streptomyces antibioticus B-546 has N2-producing activity and that most of the N2 molecules are formed via intracellular codenitrification that is induced simultaneously with denitrification.