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In Pseudomonas species and Moraxella calcoacetica, cis-cCs-muconate acts as the product inducer of catechol 1,2-oxygenase as well as -the substrate inducer for the co-ordinate induction of the enzymes catalysing its conversion into fl-oxoadipate enol lactone in Pseudomonas putida (Ornston, 1966) or into f-oxoadipoyl-CoA in Moraxella calcoacetica (Cainovas & Stanier, 1967; Canovas, Ornston & Stanier, 1967). A similar enzyme pattern, however, can also be induced with catechol. Even though it is unlikely that two compounds as different in chemical and physical properties as a diphenol and an aliphatic unsaturated, dicarboxylic acid would have similar inducing ability (Ornston, 1966) [a suggestion that has been confirmed in a strain of Pseudomonas aeruginosa (Farr & Cain, 1968) in which catechol and ciscis-muconate induce a catechol 2,3oxygenase and a catechol 1,2-oxygenase respectively], the possibility remains that catechol and cis-ci-muconate might both be effective inducers of the 1,2-oxygenase in other species. The problem was investigated by utilizing the ability of Pseudomonas aeruginosa to grow anaerobically with nitrate as the terminal electron acceptor (Stanier, Palleroni & Doudoroff, 1966). Under anaerobic conditions, the enzymic conversion of catechol into cis-is-muconate (catalysed by the irreversible 02-requiring catechol 1,2oxygenase) is prevented; it is then possible to determine the distinct inducing patterns of each compound. Experimental. Strain 8b2 of a bacterium originally isolated from sewage by enrichment with benzoate as carbon source was identified as Pseudo-