The upper part of the Santa Rosita Formation (Ordovician, Tremadocian) in the Nazareno area, Cordillera Oriental, northwestern Argentina, records the vertical passage of high-energy, shallow water platform environments to offshore settings. Eighteen trilobite species are described from this locality for the first time. Although the taxa from the lower part of the succession (Leptoplastidessp.,Asaphellussp.) are scarce and non-age diagnostic, those from the upper part include diverse assemblages partially assigned to the late TremadocianNotopeltis orthometopaZone. Systematic descriptions of several species (Geragnostus nesossiiHarrington and Leanza,G. callaveiformisHarrington and Leanza,Asaphellus jujuanusHarrington,Notopeltis orthometopa[Harrington],Mekynophrys nannaHarrington,Ceratopyge forficuloidesHarrington and Leanza,Apatokephalus tibicenPřibyl and Vanĕk) are improved, the genusNileusDalman (includingN. australisn. sp.) is first reported from the Tremadocian of western Gondwana, and new species ofAsaphellusCallaway (A. nazarenensisn. sp.),ConophrysCallaway, andApatokephalusBrøgger are described. The trilobites have their closest affinities with faunas from Norway and Sweden.Notopeltis orthometopaandMekynophrys nannaare restricted to the uppermost part of the succession, well above the first records of most other trilobites recognized.