Larvae and nymphs of Amblyomma tigrinum Koch, 1844 and Amblyomma triste Koch, 1844 were collected on Sigmodontinae rodents in Argentina. Amblyomma tigrinum was found in the phytogegraphical provinces of Chaco and Monte from the Chaco domain, while A. triste was determined in the Paranaense province of the Amazonic domain and in the Pampean province of the Chaco domain. Larvae and nymphs of A. tigrinum were found on Akodon dolores (Thomas, 1916); Akodon molinae Contreras, 1968; Akodon oenos Braun, Mares and Ojeda, 2000; Calomys musculinus (Thomas, 1913); Graomys Thomas, 1916 sp.; only larvae were found on Calomys venustus (Thomas, 1894) and Phyllotis xanthopygus (Waterhouse, 1937). Larvae and nymphs of A. triste were found on Akodon azarae (Fischer, 1829); Calomys callosus Renger, 1830; Oxymycterus rufus (Fischer, 1814) and only nymphs on Oligoryzomys delticola (Thomas, 1917). All these Sigmodontinae are new hosts for these tick species. Birds are considered crucial for the life cycle of A. tigrinum; however the results of this study show that Sigmodontinae rodents may also play a role for the maintenance of this tick in nature. This information also confirms the importance of Sigmodontinae as natural host for A. triste. Although on a wide basis the distribution of both tick species appears to be sympatric they colonize different ecological niches.