Silica bodies in the outer cells of the achene epidermis in 17 species of Eriophorum and nine species of Scirpus, some of which have been placed in Eriophorum, were revealed by sonication and/or acetolysis and examined by scanning electron microscopy. A silica body consists of a basal platform, one or more large elevated protuberances, and sometimes smaller satellite protuberances. Individual species differ in the number of protuberances, the presence/absence of satellites, the texture of the platform, and the occurrence of nodules on the larger protuberances. These features, combined with achene shape and characteristics of the anticlinal walls of the epidermal cells (undulate vs. straight, thick vs. thin, buttressed or not), are expressed in a unique combination for each species. Scirpus hudsonianus was examined for geographic and developmental variation in the microstructure of its silica bodies. All samples had similar bodies, characterized by conspicuous nodules on the protuberances and a smooth platform. Nodules were much less evident in immature achenes. Achene epidermal microstructure supports the placement of S. hudsonianus in Scirpus sect. Baeothryon and S. maximowiczii in Scirpus sect. Trichophorum. Achenes of S. hudsonianus and Scirpus cespitosus share similar silica bodies and other achene features. Achene microstructure of a late-Pleistocene fossil proved to be the same as that in Scirpus hudsonianus. Our studies establish the feasibility of identifying all species of Eriophorum by utilizing microstructural characteristics of isolated achenes.