To facilitate the cloning of DNA encoding isoquinoline degradation an assay was developed that allowed the rapid visual scoring of the isoquinoline degradation phenotype of single colonies. Transposon mutagenesis of one of the isolates. Comamonas acidovorans IQ3, was performed using Tn5, and nine Isq-mutants deficient in the ability to utilise isoquinoline as the sole nitrogen source were isolated. These mutants were also incapable of utilising the first metabolite of the isoquinoline degradation pathway, 1-hydroxyisoquinoline, as the sole carbon source. For each Isq-mutant, the EcoRI fragment containing the Tn5 insertion was cloned into pBR322. Restriction and Southern analyses of the cloned DNA revealed that of the nine Isq-mutants, six contained Tn5 insertions in a common 8.9-kb EcoRI fragment derived from the wild type, C. acidovorans IQ3. The cloned DNA thought to be involved in the degradation of isoquinoline proved to be specific when used as a probe in colony hybridization to some bacteria possessing the ability to degrade isoquinoline.