Dendryphiella salina Pugh has been grown in glucose-tryptone media differing in their ionic composition. Of the cations which have been added singly to the basal medium, potassium produces the greatest growth in terms of dry weight. Sodium stimulates dry weight production at low concentrations hut inhibits it at high concentrations. Magnesium, calcium and strontium inhibit dry weight production at all concentrations. The inhibition of dry weight production by sodium can be removed by magnesium, calcium, strontium and barium (in order of effectiveness). Under these conditions where growth is inhibited, there is a simple relationship between dry weight and mycelial potassium content. The results are interpreted in terms of the permeability of the hyphae to potassium. A number of other fungi were grown in malt extract media differing in their ionic composition. All the fungi tested have their vegetative growth inhibited by sodium; the concentration causing inhibition can differ widely from species to species. Calcium ions in all cases can reduce the extent of this inhibition. It is clear from this study that a consideration of growth media from the point of view of ionic composition may be very important in interpreting data concerning the growth of fungi.