When the eggs of the brown alga Pelvetia were grown in a gradient of the calcium ionophore A23187, they tended to form their rhizoidal outgrowths on the sides that were exposed to the higher concentration of ionophore. This result supports the hypothesis that the formation of an intracellular calcium gradient is an essential step in the polarization of these eggs; the rhizoid forms at the pole that has the higher concentration of calcium.