Klebsiella pneumoniae RS-13 and Acetobacter liquefaciens S-1, both methyl red (MR)-degrading bacterial strains, degraded N,N'-dimethyl-p-phenylenediamine (DMPD) under aerobic conditions. DMPD, a toxic and mutagenic aromatic amine, is formed during the reductive cleavage of azo dyes such as MR. The effects of physical parameters, such as temperature and aeration, and chemical parameters, such as pH and concentrations of glucose, ethanol and ammonium sulphate in the culture medium, on the degradation of DMPD by these bacteria were determined. Klebsiella pneumoniae RS-13 degraded DMPD more efficiently than A. liquefaciens S-1 under all physicochemical conditions, except in the presence of ethanol as carbon and energy sources. In addition, Kl. pneumoniae RS-13 degraded DMPD at low levels of carbon and nitrogen at pH 6-8. These results indicate that it is feasible to use Kl. pneumoniae RS-13 to completely degrade the detoxify MR under aerobic conditions.