The presence of a cytosolic pH gradient and its relation to polar tip growth was investigated in rhizoid cells of Pelvetia embryos with the use of pH-sensitive microelectrodes and by ratio imaging. Growing rhizoid cells generated a longitudinal pH gradient in which the apical cytosol was 0.3 to 0.5 units more acidic than the cytosol at the base of the cell. Treatment with a membrane-permeant weak acid, propionic acid, dissipated the cytosolic pH gradient and inhibited growth. The magnitude of the pH gradient correlated well with the rate of tip elongation. The pH gradient spatially superimposed on the cytosolic calcium gradient, and inhibition of calcium fluxes by treatment with lanthanum abolished the pH gradient and inhibited growth.