The pathways for 2,4-dinitrotoluene (2,4-DNT) and nitrobenzene offer fine illustrations of how the ability to assimilate new carbon sources evolves in bacteria. Studies of the degradation pathways provide insight about two principal strategies for overcoming the metabolic block imposed by nitro- substituents on aromatic compounds. The 2,4-DNT pathway uses novel oxygenases for oxidative denitration and subsequent ring-fission. The nitrobenzene pathway links facile reduction of the nitro- substituent, a novel mutase enzyme, and a conserved operon encoding aminophenol degradation for mineralization of nitrobenzene. Molecular genetic analysis with comparative biochemistry reveals how the pathways were assembled in response to the recent appearance of the two synthetic chemicals in the biosphere.