The carbohydrate changes which occur in Dendryphiella salina during glucose uptake over a 5-hour period in different ionic media were analysed. In all treatments the mannitol and arabitol concentration in the hyphae increased, which in 200 mM NaCl were equivalent to 85% of the glucose absorbed, and in other treatments about 30%. Polysaccharide content of the fungus either remained steady or fell, depending on the medium. From an analysis of the data, it has been possible to suggest where potassium and sodium ions might be acting in cell metabolism. Cations in the medium can affect the ability of the fungus to retain mannitol, arabitol and some unidentified compounds inside the mycelium. Sodium ions increase permeability; calcium ions decrease it. The results are discussed in terms of the effect of potassium, sodium and calcium ions on growth.