Magnesium is an essential requirement for the growth of Schizosaccharomyces pombe when grown in batch culture conditions. It is taken up by the yeast cells during growth with some fluctuations in the amount required at different phases of the growth cycle. Magnesium uptake per cell was greatest when the initial inoculum level was low. Magnesium limitation reduced the fermentative capacity of S . pombe and enhanced respiration under aerobic conditions. Oxygen uptake was also increased under Mg - limited conditions reflecting a release from catabolite repression. In the yeast Saccharomyces cerevisiae, calcium had no effect on growth rate, unlike magnesium which reduced the amount of growth dramatically. This indicated an essential role for magnesium and the non essential role of calcium in this yeast species. Magnesium limitation also altered the morphology of this yeast to a more enlarged form and inhibited cell division and budding. The growth of S . pombe was also shown to be directly affected by the availability of magnesium. When grown in a magnesium limited chemostat, an increase in cell volume was accompanied by an increase in intracellular magnesium. Also, at low growth rates, fermentation was reduced compared to that occurring at higher growth rates.