Azotobacter vinelandii was grown in continuous culture at constant dilution rate and at different molar ratios of sucrose to ammonium (C/N) in the inflowing medium. The organisms used up essentially all of the carbon and fixed nitrogen sources. Therefore, the (C/N)-ratio in the influent was the same as the (C/N)-ratio of consumption. Starting close to unity, slight increases of the (C/N)-ratio resulted in increases of cellular respiration. Concomitantly, growth yield coefficients on sucrose decreased while the total biomass stayed constant. At these low (C/N)-ratios growth was limited by ammonium with a yield coefficient on ammonium of about 0.07 g protein per mmol of ammonium. Eventually, however, upon further increasing the (C/N)-ratio, respiration as well as the yield coefficient on sucrose approached constant values while the biomass levels increased linearly. This result indicated that a transition to sucrose-limited growth had occurred. The (C/N)-ratio, above which respiration and yield coefficients on sucrose approached constancy, increased when the cultures were grown at higher oxygen tension. When the oxygen tension was higher, and at the same (C/N)-ratios, respiratory values increased, and biomass levels as well as yield coefficients decreased. The data suggest control of respiration and thus of growth yield by the ratio of sucrose to ammonium consumed. These observations infer that commencement of dinitrogen fixation kept the internal (C/N)-ratio constant and consequently respiration as well as yield coefficients on sucrose were maintained.