The energetics of growth of Escherichia coli FRAG 1 under potassium-limited growth conditions and with glucose as sole carbon and energy source were studied in the chemostat and compared with those of a mutant, FRAG 5, defective in the high-affinity potassium uptake system. The steady-state concentration of biomass decreased with increasing growth rate and was the same in both parent and mutant. For each growth rate, the rate of production of ATP was higher in the parent than the mutant strain. Under potassium-limited conditions, FRAG 1 has at least two potassium uptake systems, an inducible high-affinity uptake system and a constitutive low-affinity uptake system (Rhoads, D.B., Waters, F.B. and Epstein, W. (1976) J. Gen. Physiol. 67, 325-341). Apparently, the presence of the high-affinity uptake system in the parent leads to an energy drain. We suggest that this energy drain is due to futile cycling of potassium ions. On the basis of a mosaic non-equilibrium thermodynamic description of bacterial growth, it is concluded that the growth behaviour under potassium limitation corresponds to that expected for a catabolite limitation.