A bacterium able to grow at the expense of some isomers in a commercial surfactant preparation consisting of branched-chain dodecylbenzenesulphonate was isolated (W51), and it was identified as a Pseudomonas aeruginosa strain. A faster growing derivative was selected (W51D) after enrichment in batch culture under microaerobic conditions, using the surfactant as the sole source of carbon and energy. Strain W51D is the first microorganism reported to degrade at least 70% of a branched-chain alkylbenzenesulphonate mixture and to be resistant to high concentrations of this surfactant. The ability to degrade the surfactant was shown to be transferred by conjugation to other P. aeruginosa strains and to an Escherichia coli strain.