Maturation and fertilization of echinoderm eggs: Role of actin cytoskeleton dynamics. Starfish and sea urchin are excellent models to study the mechanisms that regulate oocyte maturation and egg activation. Hormonal stimulation of starfish oocytes and their following interaction with spermatozoa induce rapid changes of F-actin and Ca2+ increases which are prerequisites for normal fertilization and development. Fully grown oocytes isolated from the gonads of starfish contain a large nucleus (∼60-70 μm) (termed germinal vesicle, GV), which is arrested at the first prophase of meiosis. If inseminated, these immature oocytes are penetrated by additional spermatozoa. However, starfish oocytes naturally shed into the sea have already initiated the (meiotic) maturation and are normally fertilized between GV breakdown and the extrusion of the first polar body. This is considered the optimum period to ensure monospermic instead of polyspermic fertilization. By contrast, sea urchin eggs are fertilized only after being fully matured, i.e., at the end of the two meiotic divisions. Here, we provide a comparative review of the role of the actin cytoskeleton in oocyte maturation and fertilization in starfish and sea urchin. It has become increasingly evident that the exquisite regulation of the cortical F-actin is involved in nearly all aspects of the molecular events taking place during the progression of meiotic maturation and fertilization.