Herein, we provide a detailed map and a list of specimens we have examined of Eleocharis mamillata in North America. No chromosome counts have been published for North America under the name E. mamillata. However, the n  =  16 chromosome number reported under E. macrostachya from the Queen Charlotte Islands in British Columbia by Taylor and Mulligan (1968) applies to E. mamillata because the voucher specimens, originally labeled E. macrostachya, are actually E. mamillata. Eleocharis mamillata was confused until recently with E. palustris and E. macrostachya in North America and with E. palustris in Europe. It is readily distinguished from the similar E. palustris, especially by perianth bristle number, stomate morphology, and tubercle always sessile. Descriptions and images of micro-morphological characters of the epidermis of members of the E. palustris complex have been published for Europe but not for North America. We provide descriptions and images of fruits and epidermises of selected specimens of E. erythropoda, E. macrostachya, E. mamillata, and E. palustris from the central U.S. region.