Pseudomonas aeruginosa PACI grows poorly on L-lysine as sole source of carbon but mutant derivatives which grow rapidly were readily isolated. Studies with one such mutant, P. aeruginosa PAC586, supported the existence of a route for L-lysine catabolism which differes from those reported previously in other species of Pseudomonas. The postulated route, the cadaverine or decarboxylase pathway, is initiated by the decarboxylation of L-lysine and involves the following steps: L-lysine leads to cadverine leads to I-piperideine leads 5-aminovalerate leads to glutarate semialdehyde leads glutarate. Evidence for this pathway is based on the characterization of the pathway reactions and the induction of the corresponding enzymes by growth on L-lysine. The first three enzymes were also induced by growth on cadaverine and to a lesser extent by 5-aminovalerate. No evidence was obtained for the presence of pathways involving L-lysine 2-monooxygenase or L-pipecolate dehydrogenase, but another potential route for L-lysine catabolism initiated by L-lysine 6-aminotransferase was detected. Studies with mutants unable to grow on L-lysine supported the existence of more than one catabolic pathway for L-lysine in this organism and indicated that all routes converge on a pathway for glutarate catabolism which generates acetyl-CoA. Pipecolate catabolism also appeared to converge on the glutarate pathway in P. AERUGINOSA. The results suggested that the growth rate of the parental strain is limited by the rate of transport and/or decarboxylation of L-lysine. The cadaverine pathway was present, but not so highly induced, in the parental strain P. aeruginosa PACI. Pseudomonas fluorescens contained enzymes of both the cadaverine (decarboxylase) and oxygenase pathways, strains of P. putida (biotypes A and B) contained enzymes of the oxygenase pathway but not the decarboxylase pathway and P. multivorans appeared deficient in both. All these species possessed L-lysine aminotransferase activity.