SUMMARYFermentation balances determined for different substrates in batch and continuous cultures of Lactobacillus casei revealed two pathways of pyruvate conversion by this organism, a reduction to lactate and the phosphoroclastic cleavage. Pyruvate formed anaerobically from mannitol and citrate was split by the phosphoroclastic enzyme. Lactate was the main fermentation product formed during aerobic growth on mannitol and anaerobic and aerobic growth on glucose. In glucose-limited continuous cultures pyruvate conversion was dependent on the dilution rate. At low dilution rates glucose was fermented exclusively to acetate, ethanol and formate. At high rates only small amounts of acetate, ethanol and formate were formed and lactate production was maximal. Lactate dehydrogenase of L. casei had an absolute requirement for fructose-I,6-diphosphate and manganous ions. The specific activity of lactate dehydrogenase did not differ significantly at different dilution rates. It was concluded that the intracellular level of fructose-I ,6-diphosphate controlled the pathway of pyruvate conversion. In batch cultures YATP values were between 18-2 and 20.9. No evidence for oxidative phosphorylation was found. In continuous cultures YdTP values varied from 18.7 at low dilution rates to 23'5 at high dilution rates. From the dependence of YATP on the dilution rate, a maintenance coefficient of 1-52 x I O -~ was calculated. The YATP value corrected for energy of maintenance was 24.3. The possibility that the molar growth yields were erroneously high because of assimilation of growth substrate into intracellular polysaccharides, or because of energy yield from components of the medium other than the added energy source, was excluded.