Wild-type strains of Pseudomonas putida form an inducible uptake system that appears to act on beta-ketoadipate under normal physiological conditions. The system is induced by beta-ketoadipate and is represented by catabolites derived from it. Adipate is metabolized very slowly by wild-type P. putida cultures; [14C]adipate was used as an analogue of beta-ketoadipate to measure the transport activity in wild-type cells and in cells that constitutively produced the uptake system. Constitutive cells that contained high levels of the uptake system concentrated adipate to a level up to 200-fold above the concentration in the external medium. The process was energy dependent. The activity of the system with radioactive adipate was inhibited by beta-ketoadipate, by beta-ketoadipate analogues, and by some compounds (e.g., acetate, glucose) that are structurally unrelated to beta-ketoadipate; it is not known if the inhibitory effects are exerted directly by the compounds themselves or indirectly by catabolites derived from the compounds. The discovery of the beta-ketoadipate uptake system is surprising in view of earlier studies that had indicated that beta-ketoadipate does not permeate the membrane of wild-type P. putida cells. Contradictions between the former investigations and the present analysis are due primarily to the relatively high concentrations of substrate used in the earlier experiments. The existence of the beta-ketoadipate uptake system indicates that beta-ketoadipate may exist as a selective nutrient in the natural niche of P. putida and may play a determinative role in the evolution of induction mechanisms that are characteristic of fluorescent pseudomonads.