Abstract. The morphology of the obligately ectoparasitic polychaete Asetocalamyzas laonicola was studied by light and electron microscopy, and its taxonomic position was determined using molecular methods. The parasite has an extensive coelomic cavity, complete septae, and well‐developed segmental nephridia, circulatory, and digestive systems. The nervous system is rudimentary and without ganglia. The parasite's anterior region penetrates the tissues of the host, and opens into the host's body cavity. The epidermal tissues of the parasite and the host are highly integrated in the area of contact, and the parasite's cuticle is continuous with that of the host. Blood vessels of the parasite and the host may interlace in the fusion zone. The dorsal side of the parasite faces the dorsal side of the host. All parasites were males, but all hosts were females. In order to elucidate the uncertain systematic position of the parasite, molecular systematic studies were conducted. Parasite and host 18S rDNA sequences were virtually identical and revealed that both belong to the spionid cluster. These sequences differed from those of Scolelepis squamata and Scolelepis bonnieri by 2.7% and 0.9%, respectively. In addition, of seven partial sequences of the mitochondrial COI gene obtained from three parasites and four hosts, six were identical, and in one host–parasite pair, COI sequences differed by one substitution. Partial ITS2 sequences from one host–parasite pair were analyzed and also found to be similar but not identical, with two indels in a 645‐bp alignment. We conclude that the parasite is in fact a dwarf male of its conspecific spionid female host. Consequently, A. laonicola is transferred to Scolelepis (Spionidae), forming the new combination Scolelepis laonicola.

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