The effect of Dio-9, on photosynthesis and active and passive ion transport at the plasmalemma has been investigated in Nitella translucens, Tolypella intricata, and Hydrodictyon africanum. The active K influx and the coupled active Na efflux were more inhibited by this inhibitor of energy transfer in phosphorylation than was photosynthesis. The active Cl influx, the associated downhill cation influxes, and passive ion fluxes were inhibited by Dio-9, to a smaller extent than was photosynthesis. Cl influx in the light was often stimulated at concentrations of Dio-9, which inhibited photosynthesis. It was concluded that the active K influx and Na efflux require ATP, while the active Cl influx does not. Possible links between the Cl influx and electron transport and intermediates of photophosphorylation, and possible inhibition by Dio-9 of the transport ATPase in the plasmalemma are discussed.