The Plakarthriidae is a small group of flabelliferan isopods whose members have a unique, highly modified morphology. Its superficial similarity to the Sphaeromatidae has obscured the classification of this family. Plakarthrium, the only genus, is a specialized offshoot of the Flabellifera, well separated from other groups. We discuss the Plakarthriidae and its evolutionary implications and review what little is known of its natural history. The systematic section includes a taxonomic revision of the Plakarthriidae.