Investigation of a mutant strain of Pseudomonas putida NCIB 10015, strain PsU-E1, showed that it had lost the ability to produce catechol 1,2-oxygenase after growth with catechol. Additional mutants of both wild-type and mutant strains PsU-E1 have been isolated that grow on catechol, but not on benzoate, yet still form a catechol 1,2-oxygenase when exposed to benzoate. These findings indicate that either there are separately induced catechol 1,2-oxygenase enzymes, or that there are two separate inducers for the one catechol 1,2-oxygenase enzyme. Comparisons of the physical properties of the catechol 1,2-oxygenases formed in response to the two different inducers show no significant differences, so it is more probable that the two proteins are the product of the same gene. Sufficient enzymes of the ortho-fission pathway are induced in the wild-type strain by the initial substrate benzoate (or an early intermediate) to commit that substrate to metabolism by ortho fission exclusively. A mechanism exists that permits metabolism of catechol by meta fission if the ortho-fission enzymes are unable to prevent its intracellular accumulation.