Oxygen-limited (N2-fixing) chemostat cultures of Klebsiella pneumoniae supplied with a N-free medium were established by introducing low atmospheric O2 concentrations into the gas supply of anaerobic glucose-limited N2-fixing chemostat cultures; the molar growth yield for glucose and the efficiency of N2 fixation (mug N fixed/mg glucose consumed) were increased (by up to 82%) from the anaerobic values. Acetylene-reducing activity was inhibited reversibly by O2 in samples from O2-limited and anaerobic glucose-limited chemostat cultures. Oxygen uptake rates in samples from these chemostat cultures were similar, but C2-H2-reducing activity in samples from O2-limited chemostat cultures was more tolerant of low atmospheric O2 concentrations, in part because of a higher population density. In the absence of glucose, O2 was required at a low atmospheric concentration for C2H2 reduction in samples from either O2-limited or anaerobic glucose-limited chemostat cultures. The possibility is discussed that ATP generated from oxidative phosphorylation can be used for N2 fixation in K. pneumoniae.