When Aureobasidium pullulans was grown at a number of agitation rates under batch conditions, exopolysaccharide yields were dramatically reduced at high rates i.e. at least 750 rpm. Investigations with gas blending, which allowed pO(2) manipulation and control independently of the agitation rate, showed that this yield reduction was due solely to the high pO(2) levels that occurred at these agitation rates. Thus, polysaccharide production at 1000 rpm could be elevated by maintaining the pO(2) at a low level during the initial phase of the fermentation. However, both the timing of the pO(2) decrease and the level at which it was maintained were crucial for obtaining yields at 1000 rpm, similar to those observed at low agitation rates.