SUMMARYDistinct cytological changes characterize the development ofAphanomycesoogonia and oospores in alfalfa roots. Evidence for gametangial meiosis and development of dense-body vacuoles into fingerprint vacuoles is reported. The contents of the entire oogonium develop into a single oosphere. Fertilization and karyogamy trigger rapid changes in the structure and distribution of cytoplasmic components in the developing oospore. Periplasm is not formed. Fingerprint vacuoles fuse to form membrane-bounded reserve globules in the center of the oospore while lipid bodies congregate toward the oospore periphery. A thick oospore wall develops as a bipartite structure that becomes irregular in outline as the oospore cytoplasm condenses.