An early step in mammalian fertilization is species-restricted binding of sperm to the egg's zona pellucida (ZP), a thick extracellular coat that surrounds eggs. Sperm bind to the ZP of unfertilized eggs, but not to the ZP of fertilized eggs. Shortly after binding to the unfertilized egg ZP, sperm undergo the acrosome reaction, a form of cellular exocytosis that enables sperm to penetrate the ZP. Three glycoproteins, mZP1-3, constitute the mouse egg's ZP and participate in the process of fertilization. For example, sperm exposed to unfertilized egg mZP3 at nanomolar concentrations are inhibited from binding to eggs and undergo the acrosome reaction. Neither mZP1 nor mZP2 has an effect on sperm binding or the acrosome reaction. Furthermore, mZP3 from fertilized eggs has no effect on sperm binding and is unable to induce the acrosome reaction. These and other properties of mZP3 suggest that it is a receptor for sperm and inducer of the acrosome reaction. Mapping of the mZP3 combining-site for sperm suggests that it is located near the C-terminus of the polypeptide, just downstream of the ZP domain, in a region encoded by exon-7 of the mZP3 gene. This region of mZP3 is a site of positive Darwinian selection. When mZP3 exon-7 is fused to the Fc fragment of human IgG and sperm exposed to the chimeric protein, sperm are inhibited from binding to eggs. However, the chimeric protein does not induce the acrosome reaction. Therefore, polypeptide encoded by mZP3 exon-7 is necessary and sufficient for binding of mouse sperm. Mol. Reprod. Dev. 76: 933–941, 2009. © 2009 Wiley-Liss, Inc.