Climate change during the middle to late Eocene entailed a gradual global cooling from the warm "greenhouse" climate of the early Eocene to the cold "icehouse" climate of the early Oligocene.  Global cooling and increased seasonality have been attributed to a combination of tectonic and oceanographic changes, including the separation of Australia from Antarctica, the opening of the Drake Passage, and the establishment of deep-water circulation around Antarctica.  Major changes in thermohaline circulation affected global carbonate sedimentation patterns, and reflect changes in carbon cycling at this time.  Previously published d¹⁸O records from the Kerguelen Plateau in the Southern Ocean show middle Eocene cooling was punctuated by minor warming events, including the Middle Eocene Climatic Optimum (MECO).  Hence, ODP Site 748 on the Kerguelen Plateau provides an ideal high-latitude locality for studying the paleobiotic response to global ocean cooling and the MECO event.  Planktic foraminiferal assemblages were studied to characterize the diversity and extinction patterns through this time interval (~36.4-43.3 Ma).  The genus Acarinina, a mixed-layer taxon, suffers a gradual extinction following peak MECO warming.  While general trends of the long-term turnover are apparent, some of the fine-scaled details are obscured by dimorphism in some taxa.  For instance, two different wall textures can be distinguished in the morphologically plastic taxon Subbotina angiporoides, a feature reminiscent of cryptic biologic species documented in modern planktic foraminifera.  This study documents the intricacies of planktic foraminiferal turnover during in the middle to late Eocene transition, aiding to the understanding of the relationship between biotic communities and global climate change.