The mechanics of sperm entry into the oocyte were examined in rat ova fertilized in vivo. With the scanning electron microscope we observe that the spermatozoon is first associated with the oocyte by the region of the convex surface of the sperm head which is immediately anterior to the postacrosomal region. At this early stage the anterior tip of the sperm head is directed away from the oocyte surface. The spermatozoon subsequently appears to rotate so that it comes to lie on its "side." The membrane over the sperm head becomes free of microvilli and a large incorporation cone is subsequently formed. It is concluded that in the rat initial fusion of sperm and egg probably occurs between the oolemma and the sperm plasmalemma over the equatorial segment of the acrosome at the convex surface of the sperm head which is just anterior to the postacrosomal region.