Diverse and abundant organic walled microplankton have been recovered from the only known outcrops of lower Tertiary marine sediments in the Antarctic area. The 70 samples collected from the Paleogene Seymour Island deltaic complex yielded 43 genera and 74 species of dinoflagellate cysts; 2 genera and 9 species are new, and a number of new combinations and taxonomic emendations are proposed. Specimens attributable to the Acritarcha, Pterospermatales, and Chlorococcales also were noted. Sediments of the Cross Valley Formation in the Cape Wiman area are early late Paleocene in age. The exact source of samples reputedly collected from the Cross Valley Formation at the type locality in Cross Valley is uncertain. This is because faults and surface slumps in the area were only partially mapped at the time of sampling. The lower 100 m of Section 12-13 are Eocene in age, whereas the overlying 20 m are no younger than early late Paleocene. The deposits of the stratigraphically higher La Meseta Formation can be subdivided into beds of late early and middle to late Eocene age. Deposition occurred within the subenvironments of the Seymour Island Paleocene-Eocene deltaic complex. Specific paleoenvironments identified include distributary channels, interdistributary bays, lagoons, and perhaps, prodelta areas. The Paleocene dinoflagellate cyst floras are composed of cosmopolitan taxa. Eocene floras include cosmopolitan and provincial taxa. The cosmopolitan taxa have been reported from regions as widely separated as Europe, North America, and Australia. The provincial taxa belong to the hypothesized transantarctic flora that was well developed in and around Antarctica during the Eocene. The distribution of the transantarctic flora suggests that a marine seaway across West Antarctic connected the southwest Atlantic and southwest Pacific oceans during Eocene time. Rare, extra-Antarctic occurrences of some species included in the transantarctic flora suggest that such species should not be considered part of this flora, that the distribution of these taxa was bipolar, or that the complete distribution of the transantarctic flora is yet to be documented.