SUMMARYConjugation inClosteriumhas been studied using sectioned material prepared for light and electron microscopy. Prior to conjugation, cells become dense and accumulate lipid droplets. Conjugation commences in paired cells with the formation through a point of mutual contact of a circumferential strip of papilla wall material which is not necessarily centrally situated in each cell. Some microtubules are initially present, near this specialized wall which grows all around each cell but asymmetrically so that the area of papillae in contact increases and each papilla balloons out toward the other, kinking the semicells. Vesicles probably contribute material to this wall. Microtubules disappear from near the papilla as it expands; they are then often found around the older semicell wall immediately adjacent to the papilla. An enlarging vacuole is always formed in each papilla. Cytoplasmic shrinkage is first evident from an accumulation of mucilage between the plasmalemma and wall at the tip of each semicell; the terminal vacuoles collapse and disappear. During further shrinkage of the protoplast, alveolate structures are common on the plasmalemma, and the papilla wall material separating the protoplasts thins out and disappears. Meanwhile, a profound general change renders the cytoplasm far less osmiophilic and stainable for both light and electron microscopy, as it becomes relatively homogeneous and granular, revealing a considerable loss in minor cytoplasmic structures; grana in the chloroplasts become bloated. These changes are not considered processing artefacts, but may result from a breakdown of vegetative cell structures no longer needed for zygote formation. After fusion, the zygote protoplast tends to round up. Zygote maturation commences with deposition of a multilayered wall and shrinkage of the protoplast. Then the very thick zygote wall proper is slowly secreted; meanwhile, very considerable cytoplasmic condensation now renders the cell increasingly osmiophilic and dense until ultrastructural detail becomes totally obscured. Fairly consistent changes in the appearance and probably in the function of the golgi bodies were noted. The results are discussed in terms of the possible function of cell organelles and their role in the mechanics of conjugation.