This paper describes the upper Miocene-lower Pliocene and upper Oligocene to lower Miocene marine diatom floras recovered from the 227 m glacial-marine MSSTS-1 (McMurdo Sound Sediment and Tectonic Studies) drill core. These represent the southernmost occurrence known to date. Over one hundred and thirty siliceous microfossil taxa are described and/or illustrated. A provisional upper Oligocene to lower Miocene diatom biostratigraphic zonation based primarily on MSSTS-1 diatom data with supporting information from Southern Ocean DSDP Site 513A is proposed for the Ross Sea region. Recalibration of existing MSSTS-1 magnetostratigraphy, guided by diatom data, yields an age of 31 Ma at the bottom of the hole to 24.5 Ma at 115 m. The magnetostratigraphy enables determination of an average rate of sediment accumulation of 20 m/m.y. which shows minor variation. Extrapolation of this rate to 51 m yields an age of 21 m.y., consistent with the diatom data. The linear nature of the accumulation rate curve suggests no major breaks in the lower MSSTS-1 sequence, and little tectonic activity on the adjacent Transantarctic Mountain block during the late Oligocene. Micropaleontologic and sedimentologic data, however indicate at least six episodes of ice-grounding over this period. Diatom assemblage diversity and composition fluctuate between benthic-dominated shallow and planktonic-dominated deep water assemblages and probably reflect glacially induced eustatic changes. Between nearly equal 216 m and 214 m a dramatic drop in diatom diversity uphole and an increase in the benthic to planktonic diatom ratio suggests an abrupt shallowing. This event is centered at nearly equal 30 Ma (paleomagnetic data) and corresponds to the largest regression of the Cenozoic which has been attributed to a major increase in the volume of Antarctic ice. Fifteen meters of glacial-marine sediments with dropstones below the record of this nearly equal 30 Ma climatic event in MSSTS-1 indicate that ice was present at sea level in the western margins of the Ross Sea region more than 31 million years ago.