It has been proposed in recent years that the phyla Pogonophora and Vestimentifera are a derived clade of polychaete annelids. It has also been proposed that if this clade belongs among polychaetes, then the taxon name Pogonophora is misleading and should revert to a name first formulated for the group, Siboglinidae Caullery, 1914. This recommendation is adopted in this paper, and a cladistic study using terminals of «generic» rank in the former Pogonophora (including Vestimentifera) is undertaken. The purpose of this is to assess which taxon names should now be used for clades within Siboglinidae, and to provide a revised taxonomy, based on phylogenetic principles. Another major aim is to assess the position of the vestimentiferan clade within Siboglinidae. The results show that Vestimentifera is the sister group to Sclerolinum, and this clade is then sister group to Frenulata, i.e. the remaining Siboglinidae. The results suggest that all taxa within Siboglinidae that are not genera or species are redundant, except for the following: Siboglinidae is defined as the first polychaete, and all its descendants, to have an gut occluded by expanded endoderm filled with chemoautotrophic bacteria, as seen in the holotype of Riftia pachyptila Jones, 1981. Monilifera can be defined based on apomorphy-based system such that it is the first siboglinid, and all its descendants, to have rings of chaetae (uncini) in the opisthosoma, as seen in the holotype of Sclerolinum magdalenae Southward, 1972. Vestimentifera can be defined as the first siboglinid and all its descendants to have a vestimentum as seen in the holotype of Riftia pachyptia. Frenulata is defined as the siboglinid, and all its descendants, to have a mid-trunk girdle, as seen in the holotype of Siboglinum weberi Caullery, 1914. The taxa of generic rank are not defined here since their monophyly was not investigated.
