Cells of several copper-protein denitrifiers that produce nitrite reductase reduced nitrate to gaseous products but were completely or strongly inhibited by the copper chelator diethyldithiocarbamate (DDC), which did not comparably inhibit cells of cytochromeed1nitrite reductase producers. Both types of true denitrifiers released NO while reducing nitrite anaerobically in the presence of the uncoupler 3-chlorophenylhydrazonepropanedinitrile (CCCP), as seen previously only inParacoccus denitrificons. In contrast, the pseudodenitrifierRhizobium"hedysari" HCNT1, which grows as neither adenitrifier nor a fermenter, failed to reduce nitrate or nitrite or to nitrify ammonia when grown aerobically but reduced nitrite to N2O after growth at low oxygen tension even without any nitrogen oxide in the culture medium. Such oxygen-limited cells also formed N2O when incubated anaerobically with NO and hydroxylamine, and with nitrite and hydroxylamine as well, but not when anaerobic nitrite reduction (i.e., apparent production of NO) was inhibited by DDC.Rhizobium"hedysari" HCNT1 cells released no NO while reducing nitrite even in the presence of CCCP. The use of CCCP may permit differentiation of respiratory from nitrite-detoxifying denitrifying bacteria. Under aeration, but not anoxia, NO and hydroxylamine reacted to form N2O even in the absence of cells, providing a possible indicator of NO production assayable by gas chromatography.Key words: nitrite, nitric oxide, diethyldithiocarbamate (DDC), hydroxylamine, 3-chlorophenylhydrazonepropanedinitrile (CCCP).