Polar body formation is an extremely unequal cell division. In order to understand the mechanism of polar body formation, morphological changes at the animal pole were investigated in living oocytes of the starfish, Asterina pectinifera, and the amounts of cortical actin filaments were quantitatively estimated after staining the maturing oocytes with fluorescently-labeled phallotoxins using a computer and image-processing software. Formation of a bulge, which is presumed to become a polar body, and the anaphase separation of chromosomes occurred simultaneously. When the bulge became large, one group of chromatids moved into the bulge. The dividing furrow then formed and finally a polar body formed. Just at the time of bulge formation, the intensity of the fluorescence produced by the actin filaments at the top of the animal pole began to decrease, and subsequently the intensity at the top fell to half of the original value. On the other hand, the fluorescence intensity at the base of the bulge increased gradually. This actin accumulation at the base created a dividing furrow around the top of the animal pole as the bulge grew. Even when the polar body formation was inhibited mechanically, a similar pattern of actin deficiency and accumulation in the cortex near the animal pole was observed. This indicates that such regulation of filamentous actin can take place without bulging. Therefore, polar body formation is initiated by the bulging of the cortex weakened by actin deficiency and followed by contraction of the base of the bulge reinforced by actin accumulation.