The degradation of two azo dyes; Congo Red (CR) and Direct Black 38 (DB 38), in use in textile industries in Turkey, were investigated using two facultative microorganisms (Escherichia coli and Pseudomonas sp.) under anaerobic, aerobic, and microaerophilic conditions. Hundred milligram per litre of dyes and 1000 mg glucose-COD/l containing basal medium were used and were incubated for 5 and 9 days. Simultaneous biomass activity and colour removal performance, was monitored during batch assays. The effects of two different microorganisms and aerobic/anaerobic conditions on decolorization were recorded with the monitoring of colour, pH, COD, dissolved oxygen, alkalinity, and volatile fatty acids concentrations. The aromatic amines (as benzidine) arising from the metabolites of anaerobic biodegradation of dyes and the recoveries of these aromatic amines were also monitored. High benzidine recoveries indicated the accumulation of aromatic amines under aerobic conditions. The colour of the CR and DB 38 dyes were removed up to 98 and 72%, respectively, by E coli at the end of anaerobic incubation, while no decolorization occurred throughout the aerobic incubation. Under microaerophilic conditions, the azo dyes CR and DB 38 were decolorized up to 39 and 75% by E coli, respectively. In studies with Pseudomonas sp., the colour of the CR and DB 38 dyes were removed up to 100 and 83%, respectively, after 5 days of anaerobic incubation, while 76 and 74% colour removal efficiencies were observed under microaerophilic conditions. (C) 2002 Elsevier Science Ltd. All rights reserved.