The ultrastructure of the testis of the scorpion Euscorpius carpathicus during the spermiogenesis has been studied. The testis is a septate tube, that is subdivided in several compartments, limited by septa of laminar cells. In each compartment the germinal cells are at the same stage of development. Spermatogonia, spermatocytes and young spermatides are spherical cells with different size and different nuclear cytoplasmic ratio, distinguishable by a different configuration of mitochondria. They are connected by cytoplasmic bridges. In the early spermatids the nucleus adheres to cytoplasmic membrane, where a laminar acrosome developes; the mitochondria adhere to the nuclear envelope at the opposite side of the laminar acrosome. In the intermediate spermatids the nucleus shows a conical shape and chromatin condensation starts. The postnuclear cytoplasm comprises the proximal and distal centriole, surrounded by a specialization of endoplasmic reticulum, that appears as a spongy membranous complex. In the late spermatid the nucleus becomes lanceolate, with the chromatin longitudinally arranged in io nm thick lamellae. The middle piece contains 7-8 mitochondrial derivatives, forming a collar around a central flagellar tunnel. The flagellum shows an axonemal pattern 9 + O with radial filaments projected towards the center, arranged in helical fashion. The peculiarities of this long shaped sperm are analyzed and discussed in relation to the different sperm shape in the other orders of the Arachnida.