LATE TREMADOCIAN (ORDOVICIAN) CONODONTS AND GRAPTOLITES FROM THE SIERRA DE ZENTA, CORDILLERA ORIENTAL OF JUJUY, ARGENTINA. A thick heterolithic Ordovician succession lies exposed in the Zenta Range of the Cordillera Oriental, Jujuy Province, Argentina. It is mostly siliciclastic with subordinated calcareous concretions, coquinas and calcarenites, and bears a significant conodont and graptolite fauna. The sampled localities are situated on the road running from Abra de Zenta to Santa Ana, within a wide anticline structure. Exposures of gray shales and coquinas near Abra de Santa Ana, yielded a low diversity conodont fauna that includes Acodus apex n. sp., Drepanoistodus chucaleznensis Albanesi and Aceñolaza, and Drepanoistodus costatus (Abaimova), which are referred to the lower part of the Acodus deltatus — Paroistodus proteus Zone. The diagnosis of the conodont genus Acodus is emended. The A. murrayi Zone is suggested by a graptolite assemblage including Araneograptus murrayi (J. Hall), Kiaerograptus cf. K supremus Lindholm and Kiaerograptus? sp., which was collected from strata overlying the conodont-bearing levels. These conodont and graptolite faunas indicate a late Tremadocian age. Another locality on the road to Abra de Zenta, west of Santa Ana, contains Hunnegraptus cf. H. novus (Berry), Hunnegraptus spp. and Paradelograptus sp., indicating the Hunnegraptus copiosus Zone of the latest Tremadocian. The biostratigraphic succession is equivalent to that from the upper part of the Santa Rosita Formation and to the Parcha Formation exposed in other areas of the Eastern Cordillera in northwestern Argentina. The faunas contain elements of wide intercontinental distribution as well as endemic forms. The occurrence of concentrations of graptolites and lingulids showing evidence of transport in some strata indicates bottom reworking by erosive episodes in the basin.