Single nymphs of Ixodes sigelos Keirans, Clifford & Corwin, 1976 were collected in Region XI, Chile, on Akodon olivaceus (Waterhouse 1837) in Puerto Aysen (45º23´S 72º41´W), Oligoryzomys longicaudatus (Bennett 1832) in the vicinity of Lago General Carrera (46º26´S 72º00´W) and Akodon xanthorhinus (Waterhouse 1837) in Puerto Ibañez (46º17´S 71º56´W), as well as in Tucumán Province, Argentina, on Akodon spegazzinii Thomas, 1897, 3 km SW of Hualinchay (26º20´S 65º39´W). Akodon xanthorhinus and A. spegazzinii are new hosts for I. sigelos, and these are the first records of this tick from Argentina and southern Chile. The nymph of I. sigelos and nymphs of three other Neotropical Ixodes (I. neuquenensis Ringuelet, 1947, I. nuttalli Lahille, 1913, I. stilesi Neumann, 1911) bear anterior and posterior processes on palpal article I. However, I. sigelos is unique in possessing both a basis capituli that appears winged in dorsal aspect and a bifurcate posterior process on palpal article I. These collections greatly expand the known range of I. sigelos and underscore the importance of sigmodontine rodents as hosts of Neotropical ticks, especially immature stages.