In the production of alpha-amylase by Bacillus amyloliquefaciens, high aeration rates were found to be essential for good yields of the enzyme, but foaming problems resulted. The best yield obtained for a 3 l batch without control of dissolved oxygen was 4,800 U ml(-1) in 48 h at an aeration rate of 1.7 vvm with agitation at 800 rpm. Foaming, however, was severe at this aeration rate. There were indications that higher rates of aeration might give even better yields but foaming proved unmanageable. Control of dissolved oxygen tension (DOT) did enable some reduction in air consumption, thereby permitting a lesser amount of antifoam bur this was at the expense of yield. With the controller set at 40% saturation, the peak yield for amylase was only 2,550 U ml(-1), little more than half the best result obtained without control. At 60% saturation, it was not much more (2,670 U ml(-1)), indicating little or no benefit front a higher controlled saturation level. It is concluded that the control of DOT may not be advantageous in this fermentation.