A survey of mosasaur material from the Eutaw Formation (Santonian) and Selma Group (Late Santonian–Late Maastrichtian) of western and central Alabama has revealed significant stratigraphic segregation among taxa. Three distinct biozones are recognized based on this survey: the Tylosaurus Acme-zone (from the base of the Tombigbee Sand Member of the Eutaw Formation to approximately 12 m above the base of the Mooreville Chalk), the Clidastes Acme-zone (from approximately 12 m above the base of the Mooreville Chalk to the base of the Demopolis Chalk), and the Mosasaurus Acme-zone (from the base of the Demopolis Chalk to the K-T boundary at the top of the Prairie Bluff Chalk). In each case, the biozone is named for the genus comprising the majority of the specimens examined from that horizon. The previously-recognized Globidens alabamaensis Acme-zone, including the Arcola Limestone Member of the Mooreville Chalk, has been abandoned. The transition from the Tylosaurus biozone to the Clidastes biozone has been interpreted primarily as an artifact of paleoecology, reflecting habitat preference; additional support for this interpretation has come from the mosasaur fauna of the Blufftown Formation (?Late Santonian–Early Campanian) of eastern Alabama and western Georgia. The transition from the Clidastes biozone to the Mosasaurus biozone appears to represent the local expression of a reorganization within North American mosasaur communities. The uppermost portion of the Selma Group, comprised of the Ripley Formation and Prairie Bluff Chalk, has been tentatively included in the Mosasaurus Acme-zone on the basis of poorly-known faunas.