On occasion of the discovery of a new species of a marine epizoic rotifer of the enigmatic Seisonidae, from Gazi Bay in Kenya, we reassessed the classification of the group. The taxon was until now known to contain only 2 species, viz. Seison nebaliae and S. annulatus, both of which live attached to Crustacea of the genus Nebalia. The new species, Seison africanus sp. nov., was diagnosed by its species-specific trophi morphology and relatively small size. A comparison with the 2 other known seisonid species reveals a sister group relationship between S. africanus sp. nov. and S. nebaliae, based on similarities in trophi structure and, accordingly, an assumed feeding mode and relationship with their hosts. The fundamental differences between these 2 commensal sister taxa and the ectoparasitic S. annulatus prompted a reevaluation of the generic classification of these animals. Accordingly, we propose to reestablish the genus Paraseison Plate, 1887 in order to accommodate P. annulatus (Claus, 1876) (comb. nov.).