Calcium ionophore A23187 promotes ooplasmic segregation and orange crescent formation in eggs of the ascidian Boltenia villosa. When eggs were exposed to a gradient A23187 the orange crescent was induced to form in the region corresponding to the highest concentration of ionophore. This result is consistent with the hypothesis that a local increase in intracellular calcium polarizes cytoplasmic localization in the ascidian embryo.