1. Species of Pseudomonas capable of degrading arylsulphonates and detergents of the alkylbenzenesulphonate type were isolated from sewage and river water. 2. Benzenesulphinate, benzenesulphonate and toluene-p-sulphonate were rapidly degraded by these organisms with the release of the sulphonate group as sulphite; detergent homologues with a chain length up to 16 carbon atoms (4-n-hexadecyl-benzenesulphonate) also released sulphite. Sulphite oxidation to sulphate in the medium can occur non-enzymically. 3. Growth on benzenesulphonate and toluene-p-sulphonate elicited a catechol 2,3-oxygenase, which effected a ;meta' cleavage of the ring. The metabolic route for benzenesulphonate was determined as: benzenesulphonate-->catechol-->2-hydroxymuconic semialdehyde-->formate and 4-hydroxy-2-oxovalerate-->acetaldehyde and pyruvate; the enzymes catalysing these steps were all inducible. 4. Toluene-p-sulphonate was degraded via 2-hydroxy-5-methylmuconic semialdehyde to formate and 4-hydroxy-2-oxohexanoate and the latter was cleaved to propionaldehyde and pyruvate. Propionaldehyde and propionate were oxidized rapidly by toluene-p-sulphonate-grown cells but slowly by fumarate-grown organisms. 5. The specificity of the catechol 2,3-oxygenase induced by the arylsulphonates, towards catechol and the methylcatechols, varied during the purification and suggested that 3-methylcatechol was probably oxidized by a separate enzyme. Detergents of the alkylbenzenesulphonate type also induced a catechol 2,3-oxygenase in these bacteria. 6. A few isolates, after growth on benzenesulphonate, opened the ring of catechol by an ;ortho' route to form cis-cis-muconate. The enzymes to degrade this intermediate to beta-oxoadipate were also present in induced cells.