Anaerobic biodegradability and toxicity of five azo dye metabolites (Reactive Black 5, RB 5; Direct Red 28, DR 28; Direct Black 38, DB38; Direct Brown 2, DB2; Direct Yellow 12, DY-12) at parent azo dyes concentrations varying between 200 and 1800 mg l(-1) of were studied under long-term incubations such as 72 days. The metabolites of DB 38 and DB2 azo dyes at a concentration of 1800 mg l(-1), had low toxicity effect on methanogenic bacteria. TAA measurements in the beginning and last period of incubation period showed that the recovery percentages of aromatic amines for DR 28, DB 38, DB 2 and DY 12 azo dyes, were 57.6, 69.4, 66.5, and 77.1%, respectively. The low recovery of aromatic amines (57-77%) from the reduction of azo dyes was probably due to low level of cleveage of aromatic ring resulting in minor anaerobic transformation of azo dye structure. This study showed that azo dye metabolites (aromatic amines) are partly persistent under anaerobic conditions and had low toxicity under long-term incubations. (c) 2006 Elsevier Inc. All rights reserved.