One hundred and twenty bacterial isolates, from activated sludge of a treatment plant collecting wastes enriched in ethoxylated nonylphenols, were studied. Sixty isolates were selected on rich medium and 60 on mineral medium containing two nonylphenol ethoxylates as the sole carbon source. Analysis of biodiversity at the species level was performed by comparing the AluI restriction patterns of the 16S ribosomal DNA amplified by PCR from 120 isolates. The rDNA restriction analysis enabled us to cluster the isolates into 15 groups, five of which represented nearly 77% of the community. Phylogenetic analysis of five strains belonging to these main groups made it possible to assign four of them to the genera Acinetobacter, Aeromonas and Shewanella and one to the Proteus group. The analysis of plasmid content showed a high variability and suggested that horizontal gene transfer had taken place at the intraspecific, interspecific and intergeneric levels.