Azotobacter chroococcum(ATCC 7493) was grown in continuous culture with intense vortex aeration (stirring rate 1750 rpm) with up to 50% O2in the gas phase. Under these conditions the dissolved O2generally remained at zero while the cell growth rose to about twice the normally accepted value. The meaning of the term "O2-limitation" in N2-fixingA.chroococcumcultures is critically examined.