ABSTRACT: The Santonian-Maastrichtian deposits of the northern Gulf Coastal Plain of the United States (Mississippi-Alabama) are richly fossiliferous and structurally uncomplicated, offering an excellent setting in which to study calcareous microfossil biostratigraphy. Eight planktonic foraminiferal zones and seven ostracode zones are recognized in these strata, based on the stratigraphic occurrence of 45 species of planktonic foraminifera and 112 taxa of ostracodes. The planktonic foraminiferal zonation is based on standard global biozones. These include the Dicarinella asymetrica Taxon Range Zone, the Globotruncanita elevata Interval Zone, the Globotruncana ventricosa Interval Zone, the Globotruncanita calcarata Taxon Range Zone, the Globotruncanella havanensis Interval Zone, the Globotruncana aegyptiaca Interval Zone, the Gansserina gansseri Interval Zone, and the Contusotruncana contusa-Racemiguembelina fructicosa Interval Zone. Three new ostracode zones are defined, and the upper boundary of two previously defined zones are amended. These zones are the Veenia quadrialira Interval Zone (amended), the Acuminobrachycythere acuminata Interval Zone (new), the Brachycythere pyriforma Interval Zone (new), the Ascetoleberis plummeri Taxon Range Zone (amended), the Curfsina communis Interval Zone (new), the Escharacytheridea pinochii Interval Zone, and the Platycosta lixula Interval Zone. The planktonic foraminiferal biostratigraphy indicates that the age of these deposits range from early Santonian to early Maastrichtian. The Santonian-Campanian boundary is recognized on the basis of the highest occurrence surface of Dicarinella asymetrica, and demonstrates that the contact between the Tombigbee Sand Member of the Eutaw Formation and the Mooreville Chalk is diachronous. The Campanian-Maastrichtian boundary cannot be defined by calcareous microfossils due to the lack of a proxy for the newly established definition. The high-resolution biostratigraphy of the ostracodes, which are indigenous to the North American Coastal Plain, will enable subsequent studies of Late Cretaceous paleobiogeography and Gulf of Mexico-Caribbean tectonic development to be conducted.