Cultivation of Aureobasidium pullulans in medium with a low concentration of yeast extract (0.4 g/l) led to a decrease in the growth rate early in the fermentation as compared to cultivations in medium with high concentration of yeast extract. When this medium was supplemented with zinc and iron the cultivation closely resembled that obtained in medium with high concentration of yeast extract (4.0 g/l). The culture retained a high growth rate throughout the fermentation and the initiation of the mycelial to yeast (M-Y) transition and the exopolysaccharide production was delayed. In a defined medium or in defined medium without iron only a little exopolysaccharide was produced and the yeast fraction of the total biomass at the onset of the stationary phase was 22%-25%. However, cultivation in the defined medium without zinc resulted in a high production of exopolysaccharide and an increased intensity of the M-Y transition, which led to a yeast fraction of 41%.