A cladistic analysis is conducted, encompassing a series of hierarchical levels within the Sabellidae-Caobangiidae-Sabellongidae complex. Generic relationships within this complex are examined using 36 genera and 43 characters. Outgroup comparisons with the Serpulidae s.s. and Spirorbidae are initially suggested. It is argued, however, that maintaining the Spirorbidae results in the Serpulidae being paraphyletic. Therefore, the Serpulidae s.l. (including spirorbids) are used as the outgroup. The possibility of using the Terebellomorpha (= Terebellida) as an outgroup is discussed. The more inclusive relationship between the orders Sabellida and Spionida is examined, leading to the conclusion that the Spionida is paraphyletic by way of recognizing the Sabellida. Sixteen most parsimonious cladograms are found, differing only slightly in the placement of some genera. Caobangia Giard and Sabellonga Hartman, representing the monotypic Caobangiidae and Sabellongidae, respectively, fall within the Sabellidae. In order to define the Sabellidae as monophyletic, Caobangia and Sabellonga must be placed in this family. Present sabellid subfamilies are either paraphyletic or polyphyletic, except for the monotypic Myxicolinae. Revised monophyletic subfamilies are suggested, comprising a more restricted Fabriciinae and expanded Sabellinae. Caobangia is placed into the former subfamily, while Sabellonga is moved to the latter. The Myxicolinae is not recognized, with Myxicola Koch in Renier being placed in the Sabellinae. Genera of the revised Sabellidae are reviewed in light of the characters used in the cladistic analysis. Several genera are poorly defined in terms of monophyly (e.g., Oriopsis Caullery and Mesnil, Jasmineira Langerhans), while six genera are not defined by synapomorphies: Chone Kröyer, Sabellastarte Kröyer, Perkinsiana Knight-Jones, Pseudopotamilla Bush, Eudistylia Bush, and Schizobranchia Bush. Setal transformation series are discussed, suggesting the presence of regulatory mechanisms localized within a particular body region or operating throughout the thorax and abdomen. Such mechanisms appear to allow for considerable structural lability, resulting in convergence of thoracic and abdominal setal forms from different plesiomorphic states. In some cases, it also appears as though abdominal setal forms may be controlled by segment age and location along the body. Changes in thoracic or abdominal setal forms in notopodia and neuropodia within a particular segment appear to be independent of one another.