Oogenesis in the fern Adiantum flabellulatum was studied using transmission electron microscopy. The newly formed egg is closely appressed to the archegonial jacket cells. Plasmodesmata connect the egg and the ventral canal cell, but are absent between the egg and the jacket cells. During maturation, a separation cavity forms around the egg. However, a pore region, persistently connecting the egg and the ventral canal cell, is reported for the first time. Subsequently, an osmiophilic egg envelope is formed, from a combination of the products of sheets of endoplamic reticulum and osmiophilic bodies in the egg cytoplasm. The absence of this envelope from the region where the egg and ventral canal cell were formerly interconnected produces a fertilization pore, which is believed to be an entrance for the sperm penetrating the egg. In the mid stages of egg maturation, the nucleus envelope produces extensive evaginations which then appear to be sequestered into the egg cytoplasm recalling a similar phenomenon in the eggs of other ferns. Degeneration of the neck canal cells is associated with the secretion of copious mucilaginous material.