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Subantarctic sediments recovered in the Southwest Atlantic during Leg 71 provide well-preserved and often diverse silicoflagellate assemblages of middle Eocene, late Eocene-early Miocene, and late middle Miocene through Holocene age. Two holes, 511 and 513A, comprise a complete 370-meter upper Eocene to lower Miocene composite reference section which is divided into six new or revised silicoflagellate zones, in ascending order: the Mesocena occidentalis Zone, Naviculopsis trispinosa Oppel Zone, N. constricta/Dictyocha deflandrei Concurrent Range Zone, N. constricta-Corbisema archangelskiana Interval Zone, C. archangelskiana Range Zone, and Naviculopsis biapiculata Partial Range Zone. New species described from this upper Eocene to lower Miocene section include D. alta, Mesocena bispicata, and Distephanus crux paulii. Middle Eocene sediments hydraulically piston cored at Site 512 are assigned to the newly constructed Dictyocha grandis Range Zone and are subdivided into three subzones; the D. stelliformis Subzone, D. stelliformis-M. apiculata Interval Subzone, and M. apiculata Subzone. Paleomagnetic measurements and correlations of this middle Eocene section indicate deposition of the D. grandis Zone between -43.7 to 40.9 Ma. New species from this interval include D. anguinea, D. grandis, and D. stelliformis. Miocene to Holocene sediments present in hydraulically piston cored Sites 512 and 514 and the rotary cored Site 513 contain the M. circulus Zone, M. circulus/M. diodon Zone, M. diodon Zone, and Distephanus boliviensis Zone. The upper boundaries of these zones are correlated directly with magnetostratigraphy and lie in upper Chronozone 9, upper Chronozone 6, the lower Gilbert Chronozone, and the lower Gauss Chronozone, respectively. A detailed analysis was made of all silicoflagellate skeletal morphotypes present in the upper Eocene-lower Oligocene to determine the limits of species variability. Numerous sporadically occurring species were determined to be intraspecific variations of more abundant species and are placed in synonymy with these species.