The Pantex facility near Amarillo, Texas, has soil and groundwater contaminated with differing combinations of high explosives (HEs), including hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 2,4,6-trinitrotoluene (TNT). This project was concerned with direct treatment of HMX in groundwater withdrawn at this plant. Several physical and chemical treatment schemes for the treatment of HMX have been successful. However, the successful biological treatment of HMX has been limited to anaerobic environments. The objective of this work was to identify microbial consortia and amendments capable of aerobically biodegrading HMX in water. Microbial consortia and amendments employed were provided as livestock manure and soil with its indigenous flora from nearby historically contaminated sites. Possible losses of HMX by nonbiological means such as adsorption and photolysis were accounted for by appropriate abiotic experiments. Loss of the parent compound was measured by high-performance liquid chromatography, using a modification of U.S. Environmental Protection Agency (EPA) Method 8330. Results varied from no degradation to a reduction of parent HMX from 6 to 1 mg/L in 5.2 days. Evidence for biodegradation was supported by the appearance of metabolites. Metabolite identification was performed at Oak Ridge National Laboratory. Five metabolites (four intermediate and one final) were identified.