Kinetic characteristics of azo dye decolorization by Escherichia coli NO3 were determined quantitatively using C.I. Reactive Red 22 as the model azo dye substrate. The effects of substrate (dye) concentration and the environmental factors (temperature, pH, dissolved oxygen (DO) level) on the rate of bacterial decolorization were investigated. The stability and efficiency of the E. coli NO3 strain in long-term repetitive operations were also examined. The results showed that the dependence of the specific decolorization rate on azo dye concentration could be described by a conventional substrate-inhibition model. Decolorization with E coli NO3 gave a relatively high maximum decolorization activity (ca. 100 mg dye g cell(-1) h(-1)), which occurred at a dye concentration of 2000 mg 1(-1), suggesting the applicability of the strain in remediation of wastewater containing high azo dye concentrations. Dependence of the decolorization rate on temperature followed the Arrehnius equation over a normal temperature range (20-45 degrees C), while the optimal pH for decolorization was around 7-9. DO level significantly inhibited the bacterial decolorization, which could not occur if the DO level exceeded 0.5 mg 1(-1). During four repeated runs, the E. coli NO3 always maintained a higher decolorization rate than that obtained from the original run.