Climate change during the middle-to-late Eocene transition (ca. 49.0 - 33.7 Ma) 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, growth and expansion of Antarctic ice sheets, and increased seasonality during this climatic transition 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. However, previously published delta (super 18) O and delta (super 13) C records from the Southern Ocean show middle Eocene cooling was punctuated by minor warming events, including the middle Eocene climatic optimum (MECO). ODP Site 748 on the Kerguelen Plateau in the Southern Ocean provides a climatically sensitive high-latitude locality that is well suited for studying the planktonic foraminiferal response to global ocean cooling and the MECO event. Four distinct assemblages are identified within this gradual turnover, reflecting long-term variation in the thermal structure of the oceanic water column as global climate steadily cooled. Transient warming of surface and deep waters leading up to MECO warming is coincident with a high abundance of acarininids, and reflects a well-stratified water column. Following peak MECO warming ( approximately 41 Ma), surface and deep waters steadily cool coincident with the decline in acarininid abundance. Continued cooling and/or ice sheet expansion between 39.32-37.78 Ma led to a major reduction in acarininid abundance and diversity, and an increase in cold-water species at all water depths. The final stage of the turnover is denoted by the first appearance of Paragloborotalia nana, a distinct sub-thermocline species ( approximately 37.65 Ma). Two cases of dimorphism are studied within the Subbotina group, including the recognition of one previously undocumented morphotype. Though an intricate turnover in the planktonic foraminiferal community is documented, this work shows there is no overall decline in species diversity during the middle-to-late Eocene transition at ODP Site 748 on the Kerguelen Plateau.