Diatoms and silicoflageilates from netplankton collected in July 1982 off the Uruguayan and south Brazilian coasts comprised warm-water, cold-water and cosmopolitan forms. Similarity among samples (Sorensen's index) yielded consistently high figures: 0.55 to 0.87 (diatoms), and 0.57 to 1 (silicoflageilates), suggesting that the area is homogeneous from the biogeographical point of view. Sixty-one diatoms and four silicoflagellates comprise the typical winter assemblage for the zone investigated. Cosmopolitan and temperate forms, diatoms: Rhizosolenia setigera, Thalassiosira rotula, Thalassionema nitzschioides and silicoflageilates: Dictyocha fibula, D. speculum, were numerically dominant. Neritic diatoms comprised 45% of the entire inventory, while oceanic ones amounted to 16%. The remaining 39% included organisms which dwell in both types of water. Specific diversity of diatoms was high: Shannon-Weaver's values ranged between 3.4 and 4.6; for silicoflageilates the values were 0.6 to 1.5. These figures are comparable to Central Pacific phytoplanktonic diversity (diatoms, dinoflagellates, coccolithophorids and flagellates pooled), and are 3.7 to 5.7 times higher than diatom diversities recorded for the Subantarctic and Antarctic zones of the Pacific Ocean, respectively. The high diatom diversity observed in the area under study is interpreted as a result of expatriation of subantarctic and subtropical species by the Malvinas Current and Brazil Current, respectively, and to advection of brackish-water taxa due to the influence of the Rio de la Plata outflow.