CONODONT BIOSTRATIGRAPHY OF THE UPPERMOST SAN JUAN FORMATION AND THE LOWER MEMBER OF LOS AZULES FORMATION AT CERRO LA CHILCA, SAN JUAN PRECORDILLERA. The present contribution deals with a conodont association in the uppermost San Juan Formation and the lower member of the Los Azules Formation at the Cerro La Chilca section, Central Precordillera. The presence of key conodonts such as Eoplacognathus pseudoplanus (Viira), Histiodella kristinae Stouge and Microzarkodina sp. cf. M. ozarkodella Lindström suggests upper part of the E. pseudoplanus Zone. This is the first record of a conodonts association in the lower member of the Los Azules Formation. The biostratigraphic significance of “Bryantodina ” aff. typicalis Stauffer is discussed. The middle Darriwilian E. pseudoplanus Zone is introduced for the Middle Ordovician biostratigraphic chart of the Precordillera. This zone is equivalent with at least the top of the Paroistodus horridus Subzone (Lenodus variabilis Zone), E. psuedoplanus/ D. tablepointensis Zone, and H. kristinae Subzone (E. suecicus Zone).