During magnesium-deficiency continuous cultivation of the yeast Candida ethanolica in a bistat at a high Value of growth-limitation, the response of the culture to the pulsed addition of iron salts into the fermenter was studied. The culture responded only to an addition of trivalent iron ('apparent deficiency' of iron) but did not react similarly to divalent iron. Under the influence of Fe3+ the specific growth rate of the culture and the growth yield from the carbon substrate were changed. The intensity and duration of the response of the culture to an addition of trivalent iron depended on both the amount of added Fe3+ ions and on the value of growth-limitation by magnesium. It is assumed that different systems of di- and trivalent ions uptake are present in C. ethanolica. It is proposed that iron-binding compounds- siderophores (iron(3)-specific ligands for assimilation of Fe3+), probably, possessing a very high affinity for magnesium ions, can be used by the cell for Mg2+ uptake, but, due to the slow rate of this uptake, it can be detected only in media with high magnesium deficiency. (C) 2001 Elsevier Science Ltd. All rights reserved.