The effect of medium composition on the growth form of Aspergillus terreus NRRL 1960 in relation to itaconic acid fermentation has been studied. Four types of mycelial pellets were obtained under the conditions used and may be classified as (a) frayed and loose with 0.1-0.5 mm diameter (b) compact with 0.1-0.5 mm diameter (c) loose with 0.5-2.0 mm diameter and (d) compact with 0.5-2.0 mm diameter. Their respective maximum specific rates of formation and yields of itaconic acid, based on 100 g sucrose supplied, were (a) 1.25 mu mol mg(-1) h(-1) and 55-59 g, (b) 0.27-0.43 mu mol mg(-1) h(-1) and 26-38 g, (c) 0.75-0.90 mu mol mg(-1) h(-1) and 45-51 g and (d) 0.12 mu mol mg(-1) h(-1) and 10 g. The presence of Ca2+, Zn2+ and Fe2+ in the basal medium at concentrations of 23.3 mg/100 ml, 0.01 mg/100 ml and 0.006 mg/100 ml respectively were found to be adequate and crucial in obtaining the desired outgrowth for both high production rates and consistent yields of itaconic acid. The further addition of either commercial plaster of Paris or analytical-reagent-grade CaSO4, especially when activated by heating to 530 degrees C and present in excess of solubility, results in small and frayed pellets, which lead to itaconic acid yields of 55-59 g acid/100 g sugar supplied.