ABSTRACT: This contribution describes new species, genera and a subfamily (Anticytherideinae) of ostracods from the Upper Cretaceous (mid-late Campanian-Maastrichtian) marine deposits of the eastern flank of the Mississippi Embayment. New taxa assigned to the subfamily include Anticythereis dorsennus and A. slipperi; the new genus Asculdoracythereis, which includes the new species As. asculdora, As. invicta, and As. pseudoalabamensis; the new genus Frodocythereis, which includes the new species F. frodoi; the new genus Laevipellacythereis, which includes the new species L. colossus and L. laevipellis; and the new genus Tumulocythereis, which includes the new species T. incompta, T. tiberti, and T. tumulus. Previously described species assigned to the genus Anticythereis are re-evaluated, some of which are assigned to one of the new genera. Synapomorphic characters of the new subfamily include a combination of pronounced external post-ocular sulcus with corresponding pair of internal, rimmed inverted platforms and a distinctive "yin-yang" adductor muscle scar pattern. The taxa display remarkably rapid evolution. The earliest species (Anticythereis sp. 1) occurs in the planktonic foraminiferal Radotruncana calcarata Taxon Range Zone of mid- to late Campanian age and by the end of the Maastrichtian there are more than 20 species. Most species have very localized occurrences, including several that occur only at a single outcrop. Although species are found on the eastern flank of the Mississippi Embayment and the Atlantic Coastal Plain, none have been observed on the western flank of the Mississippi Embayment. Species have been assigned to the genus Anticythereis from other continents (South America, Europe and Africa), but those taxa lack distinctive features of the Anticytherideinae, which is endemic to North America. The group became extinct at the Cretaceous-Paleogene boundary.