The paederine genus Paederus s.l. ( Coleoptera : Staphylinidae) is a hyperdiverse group of some 500 species worldwide. Although the subtribe Paederina, in which the genus resides, is monophyletic the status of numerous genera and subgenera is questionable and the current classification is likely to be flawed. As part of detailed studies on the paederine fauna of China, we undertook a preliminary morphologically-based phylogenetic analysis to determine the relationships of Paederus (Gnathopaederus) and related subgenera, as a basis for a revision of the subgenus, and as an attempt to provide a better framework for future taxonomic studies on the group as a whole. The analysis was based on 66 adult morphological characters for 30 ingroup taxa, including all six species of P. ( Gnathopaederus). The results show that P. ( Gnathopaederus) and P. ( Paederus), and the genera Megalopaederus, Pachypaederus and Paederidus are monophyletic, and that P. (Eopaederus) and P. (Harpopaederus) are paraphylyetic. However, our analysis did not offer a robust solution to relationships among other clades within Paederus s.l. although it seems clear that Paederus s.l. is not monophyletic with respect to the above genera. The monophyly of P. ( Gnathopaederus) is supported by the mandibular basal medial tooth having two cuspids separated dorsoventrally. Based largely on this character state, the definition of the subgenus is refined; six species are recognized, including two new species described from China as follows: P. (G.) daozhenensis, sp. nov. from Guizhou, and P. (G.) jilongensis, sp. nov. from Xizang; three species, P. bursavacua Willers, P. yunnanensis Willers and P. solodovnikovi Willers, are newly assigned to the subgenus according to a revised subgeneric definition. New morphological characters including important secondary sexual characters and sexual dimorphism are described and male genitalia are illustrated for the first time for the type species of the subgenus, P. ( G.) szechuanus (Chapin). A key to the six species is presented and the distribution of each species is documented and discussed.
