Eighteen non-sibling mutants of Pseudomonas aeruginosa PAO were isolated that were deficient in the utilization of the β-methyl branched acid citronellic acid but not in the utilization of the unbranched n-octanoic acid (Cau mutants). These mutants are also deficient in the utilization of citronellol and citronellal. R68.45 plasmid-mediated transfer of chromosomal material has been used to map one of the mutations at about 52 min on the PAO chromosome and to show linkage of some, but not all, of the other mutations to this region. This system is of interest for bioremediation in oil spill areas since β-methyl branches block normal β-oxidation and cause recalcitrance of organic molecules present in petroleum products.