Comamonas testosteroni is a Gram-negative bacterium that can grow on steroids and polycyclic aromatic hydrocarbons (PAH) as sole carbon and energy source. Complete mineralisation of these compounds is achieved through complex metabolic pathways comprising a set of inducible enzymes. Whereas the degradation pathways for PAHs have been intensively studied, patterns of enzymes leading to ring fissions of the steroid nucleus are unclear. Several intermediates of the steroid and PAH degradation pathways have similar structures therefore the question remains of whether both classes are substrates of different degradation routes or whether some catabolic enzymes function in both pathways. Interestingly, our studies reveal that testosterone simultaneously induces the expression of steroid- and PAH-catabolising enzymes in C. testosteroni. By cloning the gene, one of these testosterone-inducible proteins (TIP1) turned out to be biphenyl-2,3-diol-1,2-dioxygenase. This enzyme has been described to convert 2,3-dihydroxybiphenyl into 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid in PAH degradation. The gene was found on a cluster encoding TIP1, three orfs, and another testosterone-inducible protein (TIP6) of unknown function. The deduced amino acid sequence of TIP1 revealed that the enzyme contains 299 amino acids (34 kDa) and shares homologies to a variety of other extradiol dioxygenases. Based on the similar catechol moieties in PAH and steroid intermediates, together with its inducibility by testosterone, it is conceivable that TIP1 functions as a steroid extradiol dioxygenase to convert steroidal secocatechols into the disecoandrostanes. Our data suggest a role of the reported TIP1 protein in both the degradation pathways for steroids and aromatic hydrocarbons.