Mature spermatozoa and spermatogenesis of Antalis entalis were studied by light, scanning and transmission electron microscopy. During spermatogenesis the spherical proacrosome arises from a single Golgi complex, migrates toward the anterior pole of the spermatid, and there differentiates to form an electron-dense collar. This is unique among examples of the simple acrosome of primitive spermatozoa. Nuclear condensation does not occur until the proacrosome and mitochondria migrate towards the future anterior and posterior poles of the spermatid. Chromatin condensation occurs by the formation of heterochromatin granules. No manchette of microtubules is observed in the course of nuclear condensation and elongation. Mitochondria achieve their mature spherical shape by decreasing in number and increasing in diameter at an early stage of spermiogenesis. No inter-mitochondrial or mitochondria-nuclear (or centriolar) junctions occur. The centriolar satellite is formed in association with the distal centriole late in spermiogenesis. Earlier, the typical `9+2' microtubular axoneme is coiled within the spermatid cytoplasm and the flagellum emerges only late in spermiogenesis. Beta glycogen granules appear late in the development of the spermatid, and are distributed in the centriolar and periacrosomal regions.