Klebsiella oxytoca ZS fermented glucose to ethanol and lactic, formic, and acetic acids, but, in contrast to many strains, accumulates pyruvic and acetic acids as the principal end products in aerobic growth conditions. This strain was grown in sulfate-limited chemostat at a fixed low dilution rate (D = 0.033 h(-1)) with glucose present in excess. When oxygen was supplied at a high level, pyruvate and acetate were produced, and the ratio NADH/NAD(+) was low (0.04) while the internal pyruvate concentration increased to 100 mu mol (g dry wt)(-1). A shortage of oxygen supply was accompanied by lactate production, an increase of the ratio NADH/NAD(+) (0.53), and an undetectable level in internal pyruvate concentration. The observed changes in LDH activity found in vitro in extracts of the cells are not strictly related to those found in vivo. In fact, the specific activity of LDH was essentially stable at 30% of dissolved oxygen tension (d.o.t.) and decreased slightly at 60% of d.o.t., whereas specific lactic acid production decreased rapidly. The in vitro LDH activity was strongly affected by the NADH/NAD(+) ratio.