We quantify sea surface temperature (SST) in the past Southern Ocean using the diatom Fragilariopsis kerguelensis that displays variable population with SST.  We explore the use of this relatively new proxy by applying it to sediment assemblages from the Sabrina Coast and Amundsen Sea. We find that Amundsen Sea and Sabrina Coast F. kerguelensis populations are different from each other.  An understanding of F. kerguelensis dynamics may help us generate an SST proxy to apply to ancient sediments.