At various times after mating, fertilized eggs were flushed from the ampulla of the oviduct of estrus hamsters. Cumulus cells were removed with hyaluronidase before fixation. After fixation, the zonae pellucidae were removed mechanically. The initial association between the fertilizing spermatozoon and the oocyte surface appears to be between the tips of oocyte microvilli and the equatorial segment. As the sperm head is incorporated, it undergoes a pronounced bend such that the anterior tip is the last portion of the sperm to be incorporated. A small incorporation cone forms at the site where the sperm head was incorporated into the ooplasm. As the sperm tail is incorporated, it forms numerous bends which distort the oolemma into a row of small elevations. From these observations, we have been able to propose a model for the mechanics of sperm entry into the ovum in vivo.