Abstract. The diuretic amiloride, an almost specific inhibitor of sodium trans port in animal cells and tissues, appears to produce a number of effects in the alga Hydrodictyon reticulatum. At 1 mmol 1 concentration it markedly reduces the influx of sodium ions (but not their active outflux), the influxes of potassium, chloride as well as of bicarbonate ions, and causes a profound decrease in the plasmalemma membrane potential. This plurality of inhibitory effects suggests that individual transport processes in the alga are mutually coupled