In the present study, sludge sample from biological treatment plant of a textile industry was acclimatized for decolourization of azo dye Direct Black 38 (DB38). A continuous culture experiment showed that the acclimatized sludge could decolourize 76% of 100 mg/l DB38. Bacterial community in the sludge was analyzed using culture-independent molecular approach to get the complete picture of its diversity. RFLP analysis of its 16S rRNA gene library divided the clones into 14 distinct groups. Phylogenetic analysis of these groups showed that they belonged to five different bacterial lineages: beta- and gamma-Proteobacteria (3 and 4 respectively), Bacteroidetes (2), Firmicutes (4) and Actinobacteria (1). The largest number of clones was found to cluster in the gamma-Proteobacteria (54%), followed by Firmicutes (19%), beta-Proteobacteria (14%), Bacteroidetes (10%) and Actinobacteria (3%). (C) 2007 Elsevier Ltd. All rights reserved.