SILURIAN MARINE ORGANIC-WALLED PHYTOPLAKTON AND MIOSPORES OF THE LOS ESPEJOS FORMATION, IN THE RIO DE LAS CHACRITAS SECTION, SAN JUAN PRECORDILLERA, ARGENTINA. Marine organic-walled phytoplankton (acritarchs and chlorophytes) and miospores were recorded from nine productive levels of the lower to middle section of the Los Espejos Formation, in the Río de Las Chacritas stratigraphic column, Central San Juan Precordillera. The marine phytoplankton is highly diversified and dominates the palynological assemblage through the section, with many species recorded for the first time in the Silurian of Argentina. Trilete spores and cryptospores assemblages are poorly diversified. In spite of the fact that most of the species display long stratigraphic ranges, palynological assemblages allow constraining the age of the Los Espejos Formation to the Ludlow. The presence of miospores, which first appeared in younger strata in other regions, could be interpreted as related to particular palaeoenvironmental conditions of this basin. The fitoplankton shows a strong correlation with coeval assemblages from Great Britain and important similarities with assemblages from Sweden, Spain and North Africa; thus demonstrating the lack of important biogeographic barriers and supporting a more cosmopolitan character of the late Silurian assemblages. The relationship between the acritarch and chlorophyte relative abundances shows important fluctuations throughout the studied section. Levels with greater abundance of clorophytes match the more prominent changes of sedimentary facies. Consequently, variations in the phytoplankton distribution could be revealing a strong dependence on local palaeoenvironmental factors.