Ergasilus von Nordmann, 1832 is the type and the most speciose genus of Ergasilidae, with more than 150 valid species. The first species of this genus, Ergasilusiheringi Tidd, 1942, was described from the Neotropical Region, from the gills of Hopliasmalabaricus (Bloch, 1794). Since then, 35 new species of Ergasilidae have been described from the Neotropics. However, a reanalysis of the available type specimens of E.iheringi revealed features that suggest that it represents a lineage independent from Ergasilus. Consequently, we propose Tiddergasilus gen. nov. (Ergasilidae: Cyclopoida) to accommodate T.iheringi (Tidd, 1942) comb. nov. The species was re-examined and redescribed based on available type specimens. Tiddergasilus gen. nov. is characterized by species with exopod of leg 4 2-segmented; antenna small and robust, with a short second segment (approximately three times shorter than the first segment), and claw with a unique morphology (short, recurved with a sub-proximal indentation in the inner margin). A phylogenetic analysis, based on a matrix of 15 taxa and 16 morphological characters in PAUP* software 4.0a152, resulted in 15 equally parsimonious trees. Based on this analysis, Tiddergasilus is closely related to a group composed of Neotropical species of Ergasilus – sharing with them antennule with 6-segments. When the highly homoplastic character “number of segments in the antennule” is removed from the analysis, Tiddergasilus forms a polytomy with the clade of genera of the former Vaigamidae, Ergasilus, and the clade Rhinergasilus+Brasergasilus+Pindapixara.
