The cytological features of the cells taking part in archegonial development and oogenesis in the fern Plagiogyria euphlebia (Kunze) Mett. were described in detail by means of light and electronic microscopy. The archegonium develops from an initial cell, which contains dense cytoplasm in contrast to the somatic cells. Two divisions of the initial cell result in a tier of three cells. The middle of which finally develops into a neck canal cell, a ventral canal cell and an egg by two unequal divisions. During maturation, the egg cell becomes progressively isolated from the adjacent cells by forming a separation cavity, a casual wall and an egg envelope. Series sections show that a fertilization pore forms in the upper egg envelope. During maturation of the egg, the nucleus produces conspicuous evaginations. The phylogenetic relationship of the fern P. euphlebia is discussed according to the cytological features in oogenesis. The cytological features observed during oogenesis support the inclusion of Plagiogyriaceae among the tree-ferns as proposed from molecular analyses.