||The biostratigraphic significance of the latest Cambrian-earliest Ordovician trilobite agnostoids from northwestern Argentina is summarized. A characterization of the faunas recognized below and above the Cambrian-Ordovician boundary is presented on the bases of information from sierra de Cajas (Jujuy Province, Eastern Cordillera; Lampazar and Cardonal Formations), río Volcancito (La Rioja Province, Famatina range; Volcancito Formation) and other localities assigned to the Parabolina frequens argentina Biozone. In addition, the stratigraphic meaning of each agnostoid species from the P. frequens argentina Zone is updated. The uppermost Cambrian is characterized by a distinctive agnostoid assemblage composed of Lotagnostus (Lotagnostus) sp. , Lotagnostus (Semagnostus) zuninoi (HARRINGTON and LEANZA), Micragnostus vilonii HARRINGTON and LEANZA, M. calviformis HARRINGTON and LEANZA, Strictagnostus? micropeltis (HARRINGTON and LEANZA), Pseudorhaptagnostus (Machairagnostus) tmetus HARRINGTON and LEANZA, Pseudorhaptagnostus (Machairagnostus) cf. tmetus HARRINGTON and LEANZA, Pseudorhaptagnostus (Machairagnostus) sp. , Gymnagnostus bolivianus (HOEK), Gymnagnostus perinflatus HARRINGTON and LEANZA, and Leiagnostus turgidulus HARRINGTON and LEANZA. This fauna is commonly recorded together with the polymeroid Beltella ulrichi (KAYSER), Onychopyge HARRINGTON, Plicatolina scalpta HARRINGTON and LEANZA, Parabolina frequens argentina (KAYSER), Parabolinella coelatifrons HARRINGTON and LEANZA, and Angelina hyeronimi (KAYSER). Based on this agnostoid-polymeroid fauna, a Pseudorhaptagnostus (Machairagnostus) - Gymnagnostus Assemblage Subzone (lower part of the P. frequens argentina Zone) is formally proposed. Agnostoid diversity drastically declines through the Cambrian-Ordovician boundary, probably as a consequence of a regional regressive-transgressive event. In the lowest Tremadoc the agnostoid records are mainly restricted to Trilobagnostus chiushuensis (KOBAYASHI) and some forms described under open nomenclature (Anglagnostus? sp. , Micragnostus sp. ). The upper part of the P. frequens argentina Zone is better defined by polymeroid trilobites. Jujuyaspis keideli KOBAYASHI is its most characteristic species.