Klebsiella sp. strain C1 isolated from activated sludge metabolized 2,4,6-trinitrotoluene (TNT) by two different pathways. The typical metabolites in the nitro group reduction pathway of TNT, such as hydroxylaminodinitrotoluenes and amino-dinitrotoluenes, were detected. Dinitrotoluenes and nitrite were also detected, possibly produced by a denitration pathway. After incubation of [U-C-14] TNT for 28 and 77 d, 2.4 and 6.24%, respectively, were released as (CO2)-C-14. This mineralization rate was higher than those reported by any other TNT degrading bacteria and might be due to the dual pathways of degradation in this bacterium.