Caenorhabditis elegans oocytes, Like those of most animals, arrest during meiotic prophase, Sperm promote the resumption of meiosis (maturation) and contraction of smooth muscle-like gonadal sheath cells, which are required for ovulation, We show that the major sperm cytoskeletal protein (MSP) is a bipartite signal for oocyte maturation and sheath contraction. MSP also functions in sperm Locomotion, playing a role analogous to actin, Thus, during evolution, MSP has acquired extracellular signaling and intracellular cytoskeletal functions for reproduction. Proteins with MSP-like domains are found in plants, fungi, and other animals, suggesting that related signaling functions may exist in other phyla.