Oogenesis in Bremia lactucae begins with a swelling of a hyphal tip. The cytoplasm within this is rich in lipid and becomes separated from that of the hypha to form an oogonium by a septum which develops centripetally. One or more antheridia then arise in a similar manner from the subtending hypha, the cytoplasm of which subsequently degenerates. The wall of the oogonium, but not that of the antheridium, thickens during maturation. Dissolution of the adposed walls of an oogonium and antheridium to permit fertilization occurs following the local discharge of cytoplasmic vesicles through the plasmalemma of both gametangia. Aggregated mitochondria may also play a part in wall dissolution. A fertilization tube which is bounded by a double unit membrane develops within the oogonium at the same time as a double membrane sphere delimits the central lipid-rich oosphere from the periplasm. After fertilization a thin wall, continuous with the walls of the gametangia, develops between the membranes surrounding the fertilization tube. Simultaneously the oospore wall begins to form between the delimiting membranes. Initially, wall development is associated with the presence of membrane bound vesicles within extensions of the inner membrane which terminate close to eroded areas of lipid droplets. Later, tubules with contents resembling the outer region of the oospore wall arise in continuity with the outer membrane. These tubules persist during the maturation of the oospore, during which the organelles of the periplasm degenerate and the contents of the spore become highly osmiophylic. Abortion can occur at stages throughout the development of the gametangia or oospore.