Sp18 is an 18 kDa protein that is released from abalone sperm during the acrosome reaction. It coats the acrosomal process where it is thought to mediate fusion between sperm and egg cell membranes. Sp18 is evolutionarily related to lysin, a 16 kDa abalone sperm protein that dissolves the vitelline envelope surrounding the egg. The two proteins were generated by gene duplication followed by rapid divergence by positive selection. Here, we present the crystal structure of green abalone sp18 resolved to 1.86 A. Sp18 is composed of a bundle of five alpha-helices with surface clusters of basic and hydrophobic residues, giving it a large dipole moment and making it extremely amphipathic. The large clusters of hydrophobic surface residues and domains of high positive electrostatic surface charge explain sp18's ability as a potent fusagen of liposomes. The overall fold of sp18 is similar to that of green abalone lysin; however, the surface features of the proteins are quite different, accounting for their different roles in fertilization. This is the first crystal structure of a protein implicated in sperm-egg fusion during animal fertilization.