The effect of adding exogenous Ca2+ to cultures of Aureobasidium pullulans which were initially entirely in the yeastlike form was examined in shake flasks and a stirred-tank reactor (STR). Since culture pH is known to affect Ca2+ availability, studies were performed at a range of starting pHs. In addition, in order to confirm the role of Ca2+, the chelating agent Ethylene Glycol-bis(beta-aminoethylether)-N,N,N’,N-Tetraacetic Acid (EGTA) and an inhibitor of Ca2+ uptake were used. In shake flasks, addition of exogenous Ca2+ led to a decrease in the proportion of the culture in mycelial form and a marked increase in biopolymer synthesis. This morphological response was apparent even at the lowest level of added CaCl2 (0.01 kg m(-3)). At high added Ca2+ (0.3 kg m(-3)), extensive precipitation occurred and a pelleted morphology resulted. Restriction of Ca2+ availability by the use of Methyl Hydroxy Butyrate (MHB) or EGTA confirmed the role of Ca2+ in the observed morphological changes. At both low (2.5) and high (6.5) initial pHs, the effects of exogenous Ca2+ were much less pronounced. Culture pH also affected the molecular mass of the biopolymer formed. In the STR, the effects of Ca2+ nutrition on morphology were less pronounced, although an effect on the molecular mass of pullulan was apparent. The possible reasons for the differing results in different experimental systems are discussed.