Morphological, distributional, and habitat data are presented for North American species of the three genera of the subfamily Neomamersinae, Neomamersa Lundblad, 1953, Arizonacarus gen.nov., and Meramecia Cook, 1963. Neomamersa lundbladi lundbladi Cook, N. lundbladi paucipora Cook, and N. hexapora Cook are redescribed based on examination of types and newly collected specimens, and six new species of Neomamersa from the United States are described, namely N. boultoni sp.nov., N. psammicola sp.nov., N. californica sp.nov., N. chihuahua sp.nov., N. neomexicana sp.nov., and N. cramerae sp.nov. Arizonacarus chiricahuensis gen.nov., sp.nov. is described from the southwestern United States. Meramecia (Meramecia) anisitsipalpis (Cook), M. (Meramecia) perplexa (Cook), and M. (Meramecia) ocularis (Cook) are redescribed based on study of types and newly collected specimens, and M. (Meramecia) occidentalis sp.nov. is described from the western United States. Meramecia (Parameramecia) multipora subgen.nov., sp.nov. is also described from the southwestern United States. Revised diagnoses, keys, and distribution maps are presented for all North American taxa. Phylogenetic relationships of Neomamersinae are discussed, leading to the conclusion that comprehensive reassessment of the families Limnesiidae and Anisitsiellidae is warranted. Consideration of available phylogenetic and distributional data suggests that Neomamersinae originated in Gondwanaland before the separation of India from the rest of the southern supercontinent. Subsequently, the clade evolved and diversified extensively in South America. Neomamersinae apparently first entered North America either by crossing a "filter bridge" during late Cretaceous, Paleocene, or Miocene times or by traversing the corridor established with the formation of the Panamanian Isthmus during the late Pliocene. The group exhibits considerable taxonomic diversity in hyporheic and groundwater habitats in the United States, and the various species represent potentially useful indicators of water quality and the impact of environmental changes on freshwater communities.