Transposon mutagenesis was performed by the method of conjugational transfer in order to identify and characterize genes encoding enzymes involved in the pathway of phenol utilization as a carbon source. Escherichia coli, which carries the Tn5-132, Was mated with Pseudomonas putida SM25 as a host. We selected a mutant that could not utilize phenol as a carbon source. Chromosomal integration of the transposon was confirmed by Southern analysis, successfully tagging the gene related to a phenol-utilizing pathway. By cell-free enzyme and genetic complementation assays, the inactivated enzyme through the mutation of the corresponding gene was identified as the catB gene, which encodes a cis,cis-muconate lactonizing enzyme.