In the process of fertilization in hens, the entering mechanism of spermatozoa into the ovum through the vitelline membrane was observed morphologically by scanning electron microscopy. Two types of ova were fertilized experimentally in vitro. One of them was taken directly from the infundibulum of the oviduct, where the ovum was enclosed only by an inner layer of the vitelline membrane. The other was obtained from laid eggs, in which the ovum was enclosed by a completed vitelline membrane. When the ovum of the first type was used, spermatozoa readily entered the ovum through gaps in the meshwork of the inner layer of the vitelline membrane. When the ovum of the second type was used, no spermatozoa could penetrate into the vitelline membrane at all. This difference was considered to be attributed to the differentiation or development of the vitelline membrane. The results of the present study may lend a morphological support to the conception that fertilization takes place in fowls at the infundibulum of the oviduct.