Oogenesis of Lygodium japonicum (Thunb.) Sw. is studied using TEM and cytochemical techniques. The early development of the archegonia of L. japonicum is similar to those of the core-leptosporangiate ferns. A mature archegonium always contains an egg, a ventral canal cell (VCC) and a binuclear neck canal cell (NCC). With the development of the egg, a separation cavity forms between the egg and the VCC. However, a pore region, with a diameter of about 3.5 μm, constantly connects the egg and the VCC. The canal cells degenerate gradually accompanied by the accumulation of mucilaginous secretions around the canal cells. PAS reaction reveals that these secretions are polysaccharide in nature. Later development of the archegonia of L. japonicum differs greatly from the core-leptosporangiate ferns. The egg nucleus becomes highly irregular and two types of nuclear evaginations are formed during oogenesis. When the egg matures, no egg envelope or fertilization pore are formed. Only a layer of amorphous material is deposited on the outer surface of the egg. This is the first observation of a fern species that has a pore region but does not result in the formation of a fertilization pore. The pore region appears to determine the formation of the fertilization pore. These observations are consistent with the hypothesized phylogenetic position of L. japonicum relative to the core leptosporangiate ferns.