Colour and COD removals of the azo dyes Congo Red (CR) and Reactive Black 5 (RB5) were individually evaluated in a sequential anaerobic/aerobic treatment system. Additionally, dye toxicity was assessed by using acute ecotoxicity tests with Daphnia magna as the indicator-organism. The anaerobic reactor was operated at approximately 27 A degrees C and with hydraulic retention times of 12 and 24 h. The aerobic reactor was operated in batch mode with a total cycle of 24 h. During anaerobic step, high colour removals were obtained, 96.3% for CR (400 mg/L) and 75% for RB5 (200 mg/L). During the aerobic phase, COD effluent was considerably reduced, with an average removal efficiency of 52% for CR and 85% for RB5, which resulted in an overall COD removal of 88% for both dyes. Ecotoxicity tests with CR revealed that the anaerobic effluent presented a higher toxicity compared with the influent, and an aerobic post-treatment was not efficient in reducing toxicity. However, the results with RB5 showed that both anaerobic and aerobic steps could decrease dye toxicity, especially the aerobic phase, which removed completely the toxicity in D. magna. Therefore, the anaerobic/aerobic treatment is not always effective in detoxifying dye-containing wastewaters, sometimes even increasing dye toxicity.