The role of magnesium ions in controlling mitochondrial structure and function in the fission yeast, Schizosaccharomyces pombe, has been studied. Results show that in control cultures of S. pombe grown in the presence of magnesium, cells follow predominantly a fermentative pathway of metabolism. The mitochondria, as visualized by electron microscopy, are large but few in number, and appear irregularly branched with indistinct cristae. On depletion of the magnesium supply, cell division ceases after about 4 hours after which there is an increase in respiration. The mitochondria increase in number and become small with well-defined cristae. On replenishment of magnesium ions mitochondria seem to fuse and there is a gradual restoration of the respiratory metabolism characteristic of control cells.