SUMMARYEvents prior to, during, and immediately following plasmogamy have been investigated inOedogonium cardiacumusing combined techniques of light and electron microscopy. Maturation of the oogonium involves the formation of an oogonial pore and the differentiation of the single egg from the larger oogonial protoplast from which it is formed. The fine structure of the sperm cell at the time of plasmogamy is described as well as the nature of its entrance into the oogonium. Cinematographic films were used to analyze the movements of the spermatozoids prior to plasmogamy and, similarly, 26 complete acts of gametic fusion were recorded and analyzed. Prior to plasmogamy the flagella‐bearing anterior extremity of the spermatozoid typically becomes elongate and is thereafter capable of flexible movements and rapid changes in shape which appear more or less independent of the rest of the cell. The sperm cell always makes initial contact with the egg surface by means of this agile, proboscis‐like, anterior end. Contact results through a combination of thrusting movements of the entire sperm cell and rapid, lateral sweeping movements of its flagellated anterior extremity against the egg surface. Gametic fusion is initiated with violent, vibrational movements of the sperm cell accompanied by loss of its flagella. Apparent fusion of the gamete membranes unites their protoplasts by a narrow cytoplasmic bridge which gradually increases in size as the sperm cell cytoplasm flows into the egg. An average time of 30.5 sec was required for complete fusion as determined from 25 typical sequences of plasmogamy recorded cinematographically. Fusion occurs even more rapidly when diploid oogonia are substituted for daploid oogonia. The entire sperm cell, with the exception of the flagella, fuses with the egg during plasmogamy. The dissimilar gamete nuclei are clearly distinguished ultrastructurally in the binucleate fusion cell. Concentrations of sperm cell mitochondria and remains of the flagellar apparatus (but no flagella) are readily recognized in the fusion cell. The fate of these and other cytoplasmic constituents of the sperm cell is discussed. Immediately after plasmogamy, and prior to karyogamy, a thin, finely fibrous layer is formed us an investment exterior to the fusion cell. Karyogamy follows shortly after plasmogamy, and both events may take place within 15 min after mixing eggs and spermatozo