The formation of the first cleavage furrow in Xenopus eggs is preceded by the appearance of a high concentration of microvilli at the animal pole in the area where the furrow is going to arise. With the deepening of the cleavage furrow, the microvilli become broader and shorter and slowly regress. This cycle of formation and regression of the microvilli is also observed during the following cleavages. Such morphological changes can be correlated to a molecular reorganization of the cell surface during cleavage, that was visualized through the use of various fluorescein-isothiocyanate (FITC)-conjugated lectins. The hypothesis that microvilli represent the site of new membrane insertion is discussed.