The manufacture and use of explosives has generated large quantities of waste material in the environment, whose primary toxic compound is 2, 4, 6-trinitrotoluene (TNT). In this study, aerobic biodegradation of TNT by a pure culture of Pseudomonas putida ATCC 12633 under batch culture enrichment was investigated. A chemostat was set up at flow rates calculated from a mathematical model and to maintain steady-state conditions at all times. Under these conditions P. putida cometabolically transformed TNT to yield 2-amino-4, 6-dinitrotoluene (2-ADNT) and 4-amino-2, 6-dinitrotoluene (4-ADNT). Azoxy dimers probably resulting from the condensation of partially reduced TNT derivatives were also found. Since TNT transformation is oxygen sensitive, TNT biodegradation was studied for 85%, 80% and 20% dissolved oxygen concentrations in the reactor. TNT reduction rates were measured by fitting a first order kinetic assay to the reduction profiles. The specific reduction rates were observed to decrease with increasing dissolved oxygen concentration.