Ultrastructural changes during spermiogenesis in the stag beetle,Aegus lavicollis, were studied with special attention to the organizing process of the centriole adjunct. In early spermatids the formation of acrosome and flagellum occurs simultaneously in the cytoplasm before nuclear condensation. The centriole adjunct first appears as a sheath surrounding a centriole in a concavity of the nucleus from which the flagellar axoneme is organized. A mass of electron-dense material accumulates progressively in the vicinity of the centriole. As nuclear transformation proceeds, electron-dense material is found at the posterior end of the nucleus and extends beyond the centriolar region to form a partial sheath around mitochondrial derivatives. Electron-dense material seems to contribute to the formation of the centriole adjunct because the former is closely attached to the latter until the latter is completely formed. In mature spermatozoa the centriole adjunct is characterized by its peculiar disposition, extending across about half the length of the flagellum and partly covering the outside of two mitochondrial derivatives.