The effect of interrupting aeration on itaconic acid fermentation by A. terreus NRRL 1960 has been studied. Under the conditions used, stopping aeration for 5 min led to a complete cessation of itaconic acid production, which was only slowly restored after 24 h when aeration was resumed. After a 5-min break in aeration and in the presence of 0.1 mM cycloheximide no itaconic acid was formed even after 3 days. It seems that, upon oxygen shortage, a rapid destruction of the itaconic-acid-producing mechanism takes place, which is restored only aerobically in a slow process involving protein synthesis. Itaconic acid fermentation is also effectively stopped by metabolic inhibitors of ATP formation, pointing to the need for biochemical energy in maintaining the fermentation. ATP is possibly needed to maintain a proper physiological (i.e. near neutral) pH inside the cells, counteracting the acid produced in the fermentation process and the low external pH (below 2.0). Inhibitors of plasma membrane ATPase have no effect on itaconic acid fermentation. This indicates that the plasma membrane might be impermeable to H+ and that ATP might rather be involved in the transport of itaconic acid out of the cell. It is suggested that insufficient aeration may lead to insufficient production of ATP which, in turn, leads to damage of the metabolic machinery by acid produced in the fermentation process.