Summary: The selectivity to K and Na of protoplast samples representing cytoplasm isolated from different regions of the hyphal filament of was investigated. Concentrations of both ions contained in successive protoplast fractions were measured. During lytic digestion, protoplasts were released first from apical regions and subsequently from progressively older regions of hyphae. A low K/Na ratio was found in protoplasts containing primarily apical cytoplasm and a high K/Na ratio was found in protoplasts originating from older regions of hyphae. The ratios were the same whether MgSO or mannitol was used as stabilizer. Absolute concentrations of both ions were higher in protoplasts of apical origin. Protoplasts stabilized in mannitol lost more ions than those stabilized in MgSO over an 8 h incubation period. Na losses were higher from apical protoplasts whereas K losses were higher from protoplasts liberated from older regions of hyphae. The addition of divalent metal cations (1·5 m-Mn or Mg) reduced losses of Na from protoplasts but did not affect loss of K. Data obtained using protoplast samples were related to those obtained for intact mycelium. Absolute losses of both ions from mycelium were lower than for protoplasts but when compared on a protein basis the data suggested that protoplasts possess properties similar to those of intact mycelium in terms of K and Na selectivity.