With chemostat cultures of Klebsiella aerogenes growing at a fixed dilution rate, initially under conditions of glucose-limitation, transition to either potassium-limitation or ammonia-limitation was found not to be a steep step function. A wide range of intermediate steady states could be established in which neither substrate was present in excess of the growth requirement. As the molar ratio of glucose: K+ in the feed medium was progressively increased, the additional glucose carbon was first converted solely to CO2. Thereafter, when the molar ratio exceeded 45, acetate, and then pyruvate and 2-ketogluconate were excreted at increasing rates. In contrast, transition to ammonia-limitation provoked an early excretion of 2-oxoglutarate and 2-ketogluconate, followed (at higher glucose input concentrations) by acetate and pyruvate. These patterns of product excretion are considered in relation to the specific nature of the growth-limitation, to probable changes in the energy charge and redox balance within the growing cells, and to the accompanying modulation of tricarboxylic acid-cycle activity.