The effect of culture dissolved oxygen (D.O.) upon biomass, scleroglucan and oxalate formation by Sclerotium glucanicum was examined in a stirred tank fermenter by oxygen enrichment. Controlling culture D.O. at 5 or 10% saturation led to increased biomass formation and decreased scleroglucan formation. The mechanism by which this occurred probably involved a non-specific diversion of C source (sucrose) away from product and towards biomass synthesis. This is at variance with the reported "stimulatory" effect of limiting D.O. levels upon scleroglucan synthesis. Controlling the culture at higher levels (20 and 30%) involved increases in impeller speed. The results of such changes were distinct from those of D.O., thus demonstrating that attempts to examine D.O. effects by means of impeller speed changes are inappropriate.