Spermiogenesis and sperm structure of the primitive acoel Paratomella rubra from the Ligurian Sea, Italy, were investigated by several methods. During spermiogenesis, after flagellar incorporation by formation of two longitudinal lateral grooves, spermatid elongation is characterized in Paratomella by the presence of four membranes encircling each axoneme plus two membranes encircling both the axonemes and the nucleus. These structures were interpreted as being three cytoplasmic canals situated one inside the other. The filiform spermatozoon has two incorporated axonemes of 9+2 type, a nucleus almost as long as the sperm cell itself, a single elongate mitochondrion, and two types of membrane-bound granules, respectively, small and gastrula-shaped, and large. Organelles are highly ordered, the sperm is bilaterally symmetrical with a single long mitochondrion on the ventral side and a regular row of large granules, for some length embedded in the nucleus, on the dorsal side. Immunocytochemical studies and the use of fluorescent nuclear dyes reveal the spatial relationships of the axonemes with the nucleus. The granules were shown by Thiery, PTA and enzyme digestion tests to contain glycoproteins and/or polysaccharides and very little protein. Glycogen particles were detected in the cytoplasm. Cells containing coiled spermatozoa undergoing resorption were found in the parenchyma. New apomorphies of the taxon Paratomella based on sperm structure are proposed: a very long nucleus, a highly bilaterally symmetrical pattern of organelles, a single long mitochondrion. These characters are not found in other Acoela, and particularly in Hesiolicium, sometimes considered closely related to Paratomella.