This study targeted the diversity of bacteria isolated from activated biomass of an ETP that treated wastewater from the dye industry. Phylogenetic analysis of 5'-partial 16S rDNA sequences revealed that the Proteobacteria (alpha, beta, and gamma) and Firmicutes were the major phyla in microbial community. Five unidentified culturable bacteria were also isolated. The activated biomass changed when acclimatized to wastewater with nitroaromatic residues. It was observed that the diversity of genera was reduced, but,the degradative capacity of sludge was enhanced. (c) 2006 Elsevier Ltd. All rights reserved.