This study documents the vertical and lateral distribution of organic-walled phytoplankton cyst assemblages from samples taken from the Demopolis-Ripley transition interval, a pelagic carbonate to marine clastic facies transition in the Upper Cretaceous Selma Group of Mississippi and Alabama. The study samples have yielded abundant and diverse assemblages of dinoflagellate, chlorophyte, and acritarch cysts. In all, 70 species of organic-walled phytoplankton cysts are treated. On the basis of the ranges of the dinoflagellate cyst species recovered from the present study samples, the Demopolis-Ripley transition interval in the study sections is correlated with the lower Maastrichtian of western Europe. Also, on the basis of the data from the present study and other unpublished dinoflagellate cyst data, the Campanian-Maastrichtian boundary in the Selma Group is shown to lie at some point within the middle one-half of the Demopolis Chalk. The application of computer-based gradient analysis programs to a Recent dinoflagellate cyst dataset from the continental shelf of South Africa has shown that gradient analysis of organic-walled phytoplankton cyst assemblages can be useful in the recognition of patterns of marine watermass distribution. Finally, the application of gradient analysis techniques, including cluster analysis, polar ordination, mean rank abundance (MBA) analysis, and average member similarity (AMS) analysis, to the Demopolis-Ripley organic-walled phytoplankton cyst assemblages has allowed the recognition of four paleoecological significant phytoplankton cyst associations. The stratigraphic distributions of these associations correspond well to the changing distributions of watermass characteristics which are likely to have accompanied the Denopolis-Ripley facies transition. / Master of Science