Ostracodes are described from the “Cerro del Fuerte Section” and neighboring localities, all in the Precordillera de San Juan (northern Argentina, San Juan Province), and from sparse faunas in Bolivia (Chuquisaca, Tarabuco Province). Additional material comes from the collection of Thomas (1905). This preliminary study gives the first detailed description of ostracode assemblages in the Upper Silurian and Lower Devonian of the South American continent. Records of other abundant lower Paleozoic (Lower Ordovician to Lower Devonian) ostracodes in Argentina, Bolivia, Brazil, Peru, and Venezuela are also reviewed. Twelve different species, belonging to Palaeocopa (Beyrichiacea and Primitiopsacea) and to Binodicopa and Podocopa (Healdiacea, Thlip-suracea, Cypridacea) are described and a new genus is established (Australobollia n. gen.). Ostracode distribution at Cerro del Fuerte attests to the presence of the Siluro-Devonian boundary within the upper Los Espejos Formation and confirms recent stratigraphical attributions mainly based on conodonts and brachiopods (Benedetto et al., 1992). The beyrichiacean Hemsiella indicates a late Ludlow to Pridolian age for the lower part of upper Los Espejos Formation. The non-palaeocope “Thlipsurella-Phanasymmetria-Ranapeltis” assemblage found in the uppermost part of the formation indicates an Early Devonian (Lochkovian) age. The present study reveals the existence of faunal links at genus (Silurian) and probably species (Early Devonian) level between South America (Argentina, Bolivia), Laurentia, Avalonia-Baltica, and northern Gondwana. For example, close affinities (e.g., Ranapeltis sp. aff. rowlandi and Thlipsurella sp. aff. ellipsoclefta) exist between the uppermost Los Espejos Formation (Argentina) and contemporaneous horizons in the North American Mid-Continent (e.g., Haragan Formation, Oklahoma; Shriver Formation, Pennsylvania). Two types of ostracode assemblages, “beyrichiacean-dominated” and “thlipsuracean-bairdiacean-cypridacean dominated,” are recognized in the Late Silurian–Early Devonian of Argentina and discussed relative to other assemblages known in Laurentia, Avalonia-Baltica, and northern Gondwana. Their paleoecological significance in relation to marine bathymetry is addressed.