This study identified microorganisms capable of using the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) or its metabolites as carbon and/or nitrogen sources under different electron-accepting conditions using C-13 and N-15 stable isotope probing (SIP). Mesocosms were constructed using groundwater and aquifer solids from an RDX-contaminated aquifer. The mesocosms received succinate as a carbon source and one of four electron acceptors (nitrate, manganese(IV), iron(III), or sulfate) or no additional electron acceptor (to stimulate methanogenesis). When RDX degradation was observed, subsamples from each mesocosm were removed and amended with C-13(3)- or ring-N-15(3)-, nitro-N-15(3)-, or fully-labeled N-15(6)-RDX, followed by additional incubation and isolation of labeled nucleic acids. A total of fifteen 16S rRNA sequences, clustering in alpha- and gamma-Proteobacteria, Clostridia, and Actinobacteria, were detected in the C-13-DNA fractions. A total of twenty seven sequences were derived from different N-15-DNA fractions, with the sequences clustered in alpha- and gamma-Proteobacteria, and Clostridia. Interestingly, sequences identified as Desulfosporosinus sp. (in the Clostridia) were not only observed to incorporate the labeled C-13 or N-15 from labeled RDX, but also were detected under each of the different electron-accepting conditions. The data suggest that C-13- and N-15-SIP can be used to characterize microbial communities involved in RDX biodegradation, and that the dominant pathway of RDX biodegradation may differ under different electron-accepting conditions. (C) 2015 Elsevier B.V. All rights reserved.