The ultrastructural features of spermatogenesis of the carnivorous sponge Lycopodina hypogea displays unusual features in sponges. The male cells derive from large archaeocyte-like cells from the sponge body, the prospermatogonia, the ultrastructure of which is described. During its first divisions in spermatogonia, then in spermatids, the prospermatogonium is already surrounded by a thin follicle made by a single cell, at the origin of the spermatic cyst in which evolve the sperm cells. The spermatid nucleus progressively elongates and its granular chromatin condenses into twisted fibers. Spermatozoa, tightly packed inside the spermatic cyst, are long and narrow monoflagellated cells. They are unusual for sponges, i.e. dart-shaped, with an anterior filament, an electron-dense, rod-like outgrowth terminating in an acrosome, an opaque cone-shaped vesicle separated from the tip of the nuclear filament by a basal plate. The proximal region of the long flagellum is located within a cytoplasmic tunnel until about one third of the cell length. In the course of spermiogenesis, a secondary envelope made by several cells extraordinarily intertwined by their pseudopodia formed around the first unicellular envelope. Species-specific symbiotic bacteria are usually present between the sperm cells. During maturation, the sperm cyst, which may be termed spermatophore, migrates from the sponge body to the sponge filaments and acquires two bundles of special forceps spicules, included in the cyst envelope. The mature spermatophores are released from the filaments. These peculiarities of spermatogenesis in Porifera are discussed in relationship with the special characters of the carnivorous sponges, Cladorhizidae, i.e. absence of aquiferous system and choanocytes.