The mechanism of unequal cleavage is one of the most intriguing subjects in cell biology. Previous studies of unequal cleavage have focused on a limited number of organisms such as yeasts, nematodes, sea urchins and annelids. The cleavage pattern of the ascidian embryo is invariant. In the ascidian embryo, the posterior-most blastomeres divide unequally in three successive cleavages. In the present study, it was shown that the ascidian embryo provides another good experimental system with which to analyze the mechanism of unequal cleavage. A novel structure, designated as CAB (centrosome-attracting body), which was found specifically in the unequally cleaving blastomeres was described. In the course of unequal cleavages, first, a thick microtubule bundle appeared between CAB and one of the centrosomes. Then with the shortening of the microtubule bundle, the nucleus with the centrosome was drawn toward CAB, situated at the posterior cortex of the blastomere. Finally, a cleavage furrow formed in the middle of the asymmetrically located mitotic apparatus and produced two blastomeres of different size, generating a smaller cell that inherits CAB. The CAB seemed to play an essential role in the unequal cleavages in the ascidian embryo.