ABSTRACTIn the phylum Nemertea, the class Hoplonemertea (former Enopla) comprises the largest number of studied species with complex spermatozoa. Asteronemertes gibsoni Chernyshev, 1991, a nemertean species having a symbiotic relationship with sea stars, is characterized by complex filiform spermatozoa. Here, spermatogenesis and spermatozoon structure in A. gibsoni have been examined using light and electron microscopy. Numerous proacrosomal vesicles of two kinds have been found in early spermatogenic cells. In spermatozoa, the elongated acrosomal complex consists of two components: a core, which is a spindle‐shaped electron‐dense acrosomal vesicle with a long anterior end, and its casing of moderate electron density that covers the acrosomal vesicle completely. The acrosomal complex is located laterally relative to the elongated nucleus. The acrosomal casing bears two rows of small, short channels between the nucleus and the electron‐dense acrosomal core. In late spermatids, the elongations of the acrosomal complex and the nucleus occur simultaneously and are mediated by numerous microtubules that disappear during the latest stages of spermiogenesis. The flagellum in spermatogenic cells and spermatozoa contains an axoneme with the usual 9 × 2 + 2 microtubular organization and is posteriorly oriented in spermatozoa. As known to date, A. gibsoni has the most modified spermatozoa among investigated Nemertea, and the complex structure of its sperm is suggested to be associated with the reproductive biology, in particular, with fertilization. Additionally, a number of similar ultrastructural features in spermatozoon organization have been found in A. gibsoni and Kurilonemertes phyllospadicola whose phylogenetic relationship was previously proven.