A phylogenetic analysis was conducted of the Nereidinae — those members of the Nereididae (Polychaeta) with pharyngeal paragnaths. We had two objectives: to test the monophyly of currently accepted genera, subgenera and informal subgeneric groupings within the Nereidinae, and, if warranted, to propose a more natural classification of the Nereidinae. Parsimony analyses were undertaken, including 52 terminal taxa from all genera and informal groupings from the large heterogeneous genera Nereis, Ceratonereis, Neanthes and Perinereis. Analyses of a character set of 52 informative characters yielded more than 10 000 equally parsimonious trees with a length of 176 steps (consistency index [CI] = 0.34, retention index [RI] = 0.66). Reweighting three times resulted in 445 most parsimonious trees with length 54.62 (CI = 0.59, RI = 0.79). Many characters widely used in nereidid systematics were found to exhibit high levels of homoplasy. The most parsimonious trees could not be rooted such that the selected ingroup, ‘Nereididae with paragnaths’, was monophyletic, causing us to reject the monophyly of the Nereidinae as currently defined. The following genera were well supported by the parsimony analyses and are newly diagnosed: Alitta, Ceratonereis, Pseudonereis, Simplisetia, Solomononereis and Unanereis. Alitta succinea, Pseudonereis cortezi, Pseudonereis noodti and Pseudonereis pseudonoodti are proposed as new combinations. The parsimony analysis supported the monophyly of neither Composetia, Neanthes, Nereis and Perinereis nor of any new groupings of remaining species presently placed in those genera. It is these poorly supported genera that comprise most species of Nereididae.
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