Intracellular Ca2+ transients occur at fertilization in the eggs of all animal species and are thought to be critical for the initiation of several events in egg activation. The rho family of small GTPases are known to organize and maintain the actin filament-dependent cytoskeleton, and rho is involved in the control mechanism of cytokinesis. In the ascidian Ciona savignyi, the first step of ooplasmic segregation observed just after fertilization is cortical contraction with egg deformation, mediated by the cortical actin filaments. C3 exoenzyme, a rho-specific inhibitor, did not affect the pattern of [Ca2+]i transients in the ascidian egg, but inhibited ooplasmic segregation and cytokinesis at the first cleavage. Injection of inositol 1,4,5-trisphosphate or treatment of Ca2+ ionophore induced deformation of the egg and extrusion of the first polar body, but these phenomena did not occur in the C3 exoenzyme-injected egg. These results suggest that rho proteins are involved in egg deformation, ooplasmic segregation and cytokinesis downstream of the [Ca2+]i transients.