The effect of yeast extract on morphology and exopolysaccharide formation of Aureobasidium pullulans was followed quantitatively in batch cultivations in 2 1 fermenters. When the fungus was grown at controlled pH 6·5 or 3·5 or uncontrolled with initial pH 6·5 in media with glucose (10 gl−1) and surplus of ammonium the formation of yeast-like cells and yeast-like growth were controlled by the initial concentration of yeast extract (varied from 0·4 to 4 g l−1). With sufficient yeast extract added the growth form was filamentous until glucose became exhausted at the onset of stationary phase. Formation of yeast-like cells was a prerequisite of formation of exopolysaccharide, the initiation of which was correlated with the initiation of yeast-like growth. Exopolysaccharide was produced in media as well with a surplus of ammonium as without ammonium.