An Escherichia coli strain was mutated by random mutagenesis via exposure to UV light. The resulting mutants were acclimated in azo-dye-containing media to select for highly efficient and stable decolorization activity on two azo dyes (Reactive Black B and Reactive Red 22). Two mutant strains (namely, E coli UVT1 and E. coli UV68) were identified and their specific decolorization rates for the azo dyes were up to 3-fold faster than that of the mother strain. The mutant strains also displayed up to 7-fold increase in total decolorization efficiency, which is represented by the time required for 50% color removal. Decolorization characteristics of the mutants were investigated in terms of their dependence on dye concentration, glucose concentration, and environmental conditions (e.g., temperature, agitation rate, etc.). The mutant E coli UVT1 exhibited optimal color removal activity under high dye concentrations and was less sensitive to oxygen inhibition. The decolorization performance of the UVT1 strain was superior to that of the other mutant strain UV68. Decolorization by the mutants was shown to be an enzymatic event, while contribution of metabolism-independent biosorption to the color removal was negligible. Kinetic analysis by Monod-type model suggests that the mutation may lead to an enhanced accessibility of the dye substrate to the bacterial decolorizer. The inhibitory effect of glucose on decolorization was observed and seemed to be closely associated with sharp pH decreases due to formation of acidic metabolites while using glucose for cell growth and decolorization.