Diverse, siliceous microfossil assemblages—including marine diatoms, silicoflagellates, ebridians, endoskeletal dinoflagellates, chrysophyte cysts (archaeomonads), radiolarians, and sponge spicules—were recovered from Seymour Island. Their stratigraphic occurrence is documented from the ~l,400-m-thick section of the López de Bertodano (upper Campanian into lower Paleocene) and Sobral (lower Paleocene) Formations. These units consist of detrital silt and fine sand deposited in a quiet shelf environment. The following new diatoms are proposed: Coscinodiscus sparsus, Gladius antarcticus, Gladius antarcticus f. alta, Hemiaulus huberi, Hemiaulus seymouriensis, Pterotheca minor, Pterotheca trojana, and Wittia macellarii. The microfossil assemblages compare well with floras of similar age from diatomites in the Ural Mountain region of the Soviet Union and from Deep Sea Drilling Project (DSDP) Hole 275 in the southwest Pacific. The present work documents the fourth occurrence of Campanian/Maastrichtian, and the first of Danian diatoms and silicoflagellates from the Southern Hemisphere. High sedimentation rates have preserved a detailed record of Late Cretaceous and early Paleocene siliceous microfossil evolution and extinction. Silicoflagellate assemblages show an abrupt composition change from Lyramula-dominated to Corbisema-dominated floras a few meters above a resistant glauconitic sandstone within the upper López de Bertodano Formation. The abundance of diatom resting spores increases from ~5 percent in the uppermost Maastrichtian to ~35 percent a short distance above this glauconite and continues at these high values through the lower Paleocene. These and other fossil data suggest that the Cretaceous/Tertiary (K-T) boundary is situated a few meters above the glauconitic sandstone. Unlike most other known K/T sections, clastic sedimentation was continuous across the boundary. Most Cretaceous diatom species (as much as 84 percent) continue into Tertiary beds. This suggests that the extinction event responsible for devastating the other major groups of Cretaceous plankton did not affect the diatoms; resting spore formation may have aided their survival.