A total of fifteen bacteria from activated sludge were isolated for th eir ability to grow on aromatic substrates as the sole source of carbon and energy. They were identified as members of the genera Pseudomonas and Aeromonas. Four P . putida spp. To-1, To-3, To-5, Na-13 and one A. caviae sp.To-4 capable of good growth on toluene were selected for fu rther study. All five bacteria were capable of growth on toluene, benzoic acid, m-toluic acid, ethylbenzene, pseudocumene, m - and p-xylene and m - and p -chlorophenol. A. caviae To-4 alone was capable of growth on phenol and naphthalene. The growth of these bacteria was investigated under different cultural conditions - solid culture, liquid culture, directly added toluene and in the presence of other carbon sources. Growth was b e tter when toluene was supplied in the vapour phase. The toxic effect of directly added toluene was alleviated by the addition of a readily metabolizable carbon source and by the acclimation of the cells to toluene. Strains To-4 and To-5 were selected as representative strains of the genera to investigate the removal of toluene. Both of the strains behaved similarly in respect to the utilization of toluene and the utilization of toluene was dependent on the availability of toluene and oxygen. The optimum pH for growth of all the isolates in the presence of toluene was between pH 5.8 to pH 7.4. All five isolates showed greater meta activity thanortho activity in degrading toluene. However following growth on non-aromatic substrates the Pseudomonas spp. showed meta activity and the Aeromonassp. showed ortho activity. A large plasmid band (85 - 120 kb) was detected in all the P seudomonasspp. following growth on aromatic and non-aromatic substrates. The stra in A. caviae To-4 did not carry any plasmid following growth on nonaromatic substrates. This stra in carried a large plasmid (85 - 120 kb) only when grown in the presence of toluene. The identification of the plasmid was confirmed by dot blot hybridization using a TOL probe. Restriction analyses of the plasmid DNA of A . caviae To-4 and P . putida To-5 were compared to that of the TOL plasmid of P . putida NCIB 10432 and the presence of specifi sequences of the TOL plasmid were confirmed by Southern hybridization using both a digoxigenin-labelled and a radioactively labelled probe. The strains To-4 and To-5 carried an identical TOL plasmid which contained some of the sequences of the TOL plasmid of P . putida NCIB 10432.