Zoosporogenesis in the oomycete,Pythium proliferum de Bary initially involves a condensation of cytoplasm at certain hyphal tips and the subsequent enlargement of these hyphal tips to form sporangia. Deposition of a septum at the base of the sporangium and initiation of an apical papilla are followed by cleavage of the sporangial cytoplasm. Packets of presumptive mastigonemes as well as flagella are recognized in the cytoplasm at this time. Subsequently, cleavage vesicles at the periphery of the sporogenic cytoplasm fuse with the plasmalemma thereby emptying their fibrous contents into the space between the sporogenic cytoplasm and the sporangial wall. It is felt that this fibrous material is instrumental in developing the internal pressure necessary within the sporangium to cause discharge of the sporogenic cytoplasm into an evanescent vesicle wherein delimitation of the zoospores is completed. The formation of the spore vesicle from the multilayered apical cap of the papilla is described here for the first time in the Oomycetes and a new term, “vesiculogen”, is suggested for this structure. Aspects of centriole replication within vegetative hyphae, papilla formation, and morphogenesis of various vesicular inclusions are also described in this study.