45Ca2+ fluxes across the plasma membrane of zygotes of the fucoid alga, Pelvetia fastagiata (J. Ag.) De Toni, were studied in artificial sea waters of various potassium concentrations. Except for two cases, hyperpolarization of the cell membrane (with low [K+]) increases, and depolarization (with high [K+]) decreases the influx of Ca2+ over the range of [K+] studied (1–100 mM). The fractional increases of influx during hyperpolarization are close to the fractional increases in membrane potential but the decreases during depolarization are much smaller than those in membrane potential. In two anomalous cases, the influxes of 45Ca2+ at a potassium concentration of 30 mM were about 20% higher than the control value instead of being 10% lower. The effluxes of 45Ca2+ are increased by both hyperpolarization and by depolarization. On balance (and excepting the two anomalous cases) the net result of hyperpolarization should be to increase and that of depolarization to decrease intracellular [Ca2+].