This contribution provides the first integrative study of Cyamioidea, a group of bivalves largely neglected in the past.By combining information obtained from shell morphology, gross anatomy, histology, reproductive biology and DNAsequencing, the distinctive characters of Cyamioidea are determined, and the taxa belonging to this superfamily are revised. Gaimardioidea, previously considered as a distinct superfamily, is reduced to family level (Gaimardiidae),which, along with Cyamiidae, are assigned to Cyamioidea. All studied cyamioideans share a common reproductive character: the presence of a true follicular epithelium surrounding each developing oocyte, which persists afterspawning, surrounding the developing embryos and participating in their anchorage to the gill filaments. Several morphological and anatomical characters support the distinction of Gaimardiidae and Cyamiidae. Based on thatinformation, the placement of Gaimardia and Kidderia in Gaimardiidae, and that of Cyamiocardium, Cyamiomactra, Cyamium, Heteromactra, Pseudokelly, Ptychocardia and Reloncavia in Cyamiidae, is confirmed. Jukesena, historicallyplaced in Veneroidea, is here allocated to Cyamiidae. Neoleptonidae, previously also attributed to Cyamioidea, are reallocated to Veneroidea.Fil: Zelaya, Diego Gabriel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Biodiversidad y Biología Experimental; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Güller, Marina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Biodiversidad y Biología Experimental; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ituarte, Cristian Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentin
