Pseudomonas cepacia G4 expresses a unique toluene orth-monooxygenase (Tom) that enables it to degrade toluene and trichloroethylene (TCE). Transposon mutants of G4 have been isolated that constitutively express Tom. Two fixed-film bioreactor designs were investigated for the exploitation of one such constitutive strain (G4 PR1) in the degradation of vapor-phase TCE. DNA probe analysis indicates that the genes responsible for the toluene catabolic pathway are located on a large plasmid of G4 (> or = 150 kb) termed pG4L. The authors propose that pG4L will serve as a archetype for a new class of catabolic plasmid known as Tom, which encodes an ortho-hydroxylation pathway for the degradation of benzene, toluene, o-xylene, cresols, and phenol.