A fast hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)-degrading [28.1 mumol h(-1) g (dry weight) cells(-1); biomass, 0.16 g (dry weight) cells(-1)] and strictly anaerobic bacterial strain, HAW-1, was isolated and identified as Clostridium bifermentans using a 16S-rRNA-based method. Based on initial rates, strain HAW-1 transformed RDX to hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX) with yields of 56, 7.3 and 0.2%, respectively. Complete removal of RDX and its nitroso metabolites produced (%, of total C or N) methanol (MeOH, 23%), formaldehyde (HCHO, 7.4%), carbon dioxide (CO2, 3.0%) and nitrous oxide (N2O, 29.5%) as end products. Under the same conditions, strain HAW-1 transformed MNX separately at a rate of 16.9 mumol h(-1) g (dry weight) cells(-1) and produced DNX (25%) and TNX (0.4%) as transient products. Final MNX transformation products were (%, of total C or N) MeOH (21%), HCHO (2.9%), and N2O (17%). Likewise strain HAW-1 degraded TNX at a rate of 7.5 mumol h(-1) g (dry weight) cells(-1) to MeOH and HCHO. Furthermore, removal of both RDX and MNX produced nitrite (NO2-) as a transient product, but the nitrite release rate from MNX was quicker than from RDX. Thus, the predominant pathway for RDX degradation is based on initial reduction to MNX followed by denitration and decomposition. The continued sequential reduction to DNX and TNX is only a minor route.