The spermatogenesis of Allostoma sp. and the organization of mature spermatozoa are described by means of transmission electron microscopy. All stages of sperm development were observed in mature testes, which are bordered by a layer of fibrous connective tissue. The primary spermatogonium produces 64 spermatids, grouped as a cytophore. The process of spermiogenesis occurs at the periphery of the cytophore, when the sperm body is formed and released at complete maturation. The mature spermatozoon is a flattened, filiform structure. It is aciliary. Its anterior end is not distinguishable from the posterior one in the testis. The nucleus, showing an approximately rectangular cross section, is situated centrally and the chromatin is densely packed. A membranous cylinder, located on each side of the nucleus, contains conspicuous stacks of clear thick-envelope vesicles, and a long mitochondrial derivative placed peripherally. The vesicles show contacts with the cylinder membrane and the outer sperm membrane. Their supposed Golgi-derived content may thus be released into the external medium. A hypothesis is proposed about the role of these vesicles in sperm motility.