Escherichia coli carrying the entire nif gene cluster from Klebsiella pneumoniae on a multicopy plasmid becomes more O-2-resistant in a N-free medium as a result of the integration of the nif gene cluster into the chromosome and the loss of the plasmid (H.lwahashi and J.Someya, Biochem. Biophys. Res. Comm. 1990, 168 : 288-294). Our purpose is to characterize the physiological reason why the strain became O-2-resistant by measuring the levels of nif proteins in cells under microaerobic conditions. The O-2-resistant strain had a higher amount of NifH and a lower amount of NifL under microaerobic conditions (compared to that under anaerobic conditions), while the parent strain showed the opposite characteristics. Thus, the biochemical mechanism of the O-2-resistant strain is attributed to the strain＇s ability to synthesize and maintain a high amount of NifH and a low amount of NifL under microaerobic conditions.