N-2-fixing continuous cultures of Azotobacter vinelandii ATCC 9046 were carried out under various dissolved oxygen tensions (2, 25 and 50% air saturation) and, for each of these oxygen concentrations, the culture pH was controlled at 6.2 and 7.4. The culture pH exerted a profound influence on the specific consumption rates of glucose and oxygen and an the growth yields. Parallely, the total metabolic cost far N-incorporation was affected : for incorporating a given amount of N an extra glucose consumption of more than 70% took place when the culture pH was changed from 7.4 to 6.2. This effect was observed when the dissolved oxygen tension in the cultures was 25 or 50% but was lass pronounced when it was 2% air saturation.