Spermiogenesis in Vampirolepis microstoma begins with the formation of a nuclear cone and a differentiation zone. This is delimited at the front by arched membranes, bordered by cortical microtubules, and contains two parallel centrioles linked together at their bases by electron-dense, amorphous material. The nuclear cone elongates, becomes filiform, and migrates into the spermatid body. Later, one of the centrioles gives rise to a flagellum that grows at the same pace as the cortical microtubules. Subsequently, 6 crested bodies form and the old spermatid separates from the residual cytoplasm. The mature V. microstoma spermatozoon is filiform and lacks mitochondria. Its anterior end exhibits six crested bodies 100 to 200 nm thick of unequal lengths. The axoneme is of the 9ϩ''1'' pattern. The cortical microtubules are spiralized and make an angle of about 20 to 30°to the spermatozoon axis, except at their posterior extremity where they become parallel to this axis. The nucleus is an electron-dense cord coiled in a spiral around the axoneme. The cytoplasm is slightly dense but contains many electron-dense granules in regions III, IV, and V of the spermatozoon. The presence of centrioles linked together at their bases by electron-dense material has never, to our knowledge, been reported in a Platyhelminth. Likewise, a nuclear migration, right from the beginning to the end of spermiogenesis, has never been described in a cestode. In addition, we observe for the first time the existence of six crested bodies in a cestode from a Mammal.