Dendryphiella salina was grown at 21° C in two media, identical except that one contained potassium labelled with 42K. The cultures were transferred to 2° C, the fungus separated from the medium, the radioactive mycelium placed in the non-radioactive medium and the efflux of 42K measured. Under these conditions the rate of dry weight increase is reduced by at least 75%. The fungus behaves as a multicompartment system with regard to potassium with at least 75% of the ion being in the slowest-exchanging compartment. The efflux from this compartment into a medium containing 2.5 mM potassium is of the order of 0.14 p moles cm−2 sec−1, being at least doubled when 50 mM sodium is present. Addition of 10 mM calcium in the presence of sodium reduces the efflux to a value close to that obtained when sodium is absent.