A preliminary phylogenetic analysis of all described 49 species (plus 2 undescribed) from the 18 currently accepted genera of Nerillidae (Polychaeta, Annelida) is presented. A restricted analysis including fewer taxa yields a more resolved cladogram. Nerilla branches off basally as sister group to the other nerillids and the clade Meganerilla – Xenonerilla branches off next. Three additional clades are supported by several character transformations: Psammoriedlia – Bathychaetus – Paranerilla, Afronerilla – Thalassochaetus – Troglochaetus and Aristonerilla – Micronerilla – Trochonerilla. Mesonerilla and Nerillidium are paraphyletic. A larger generic complex contains Nerillidium and four other genera. Based on the phylogenetic results and morphological similarities, three of the twelve monotypic genera are here synonymized: Xenonerilla is synonymized with Meganerilla, Bathychaetus with Psammoriedlia, and Akessoniella with Nerillidium. Nerillidium simplex is reassigned to Troglochaetus, and Mesonerilla diatomeophaga is reassigned to Leptonerilla. An emended diagnosis of the family, a key to genera, and a detailed account of valid genera are presented.
