Klebsiella aerogenes NCTC 418 was grown in chemostat cultures (D = 0.17 hr-1; pH 6.8;35 degrees C) that were, successively, carbon-, sulphate-, ammonia-, and phosphate-limited, and which contained as the sole carbon-substrate first glucose, then glycerol, mannitol and lactate. Quantitative analyses of carbon-substrate used and products formed allowed carbon balances to be constructed and direct comparisons to be made of the efficiency of substrate utilzation. With all sixteen cultures, carbon recoveries of better than 90% were obtained. Optimum utilization of the carbon substrate was invariably found with the carbon-limited cultures, the sole products being organisms and carbon dioxide. But the extent to which excess substrate was over-utilized varied markedly with both the nature of the growth-limitation and the identity of the carbon-substrate. In general, sulphate-, ammonia-, and phosphate-limited cultures utilized glycerol more efficiently than mannitol, mannitol better than lactate, and glucose least efficiently. Glucose-containing cultures also synthesized some extracellular polysaccharide. When the carbon source was in excess, a range of acidic compounds generally were excreted. Sulphate-limited cultures, growing on glucose, excreted much pyruvate and acetate, whereas similarly-limited cultures growing on glycerol, mannitol or lactate produced only acetate. Ammonia-limited cultures invariably excreted 2-oxoglutarate and acetate, whereas phosphate-limited cultures produced gluconic acid, 2-ketogluconic acid and acetate, when growing on glucose, but only acetate when growing on mannitol or lactate. From the rates of substrate and oxygen consumption, and the rates of cell synthesis, yield values for both substrate and oxygen were calculated. These showed different trends, but were similar in being highest under carbon-limitation and substantially lower under all other limitations. The physiological significance of these findings, and the probable nature of the regulatory mechanisms underlying "overflow metabolism" are discussed.