New remains of Megathericulus patagonicus Ameghino, 1904, the first additional material of this species found since erection of the species, are described. Two individuals from separate localities, the Cerro Guenguel and Arroyo Pedregoso localities in southern Argentina, are represented. The specimens were recovered from the Río Mayo Fm. and are assigned to the middle Miocene Friasian South American Land Mammal Age. Radiometric dating from the Cerro Guenguel locality indicates an age somewhat younger than 11.8 Mya. Megathericulus patagonicus is the earliest certain megatheriine and the new remains provide information on primitive megatheriine features. Among these are that lower molariforms are mesiodistally compressed, as in the palate of the type specimen; ventral margin of dentary is prominently bulged; angular process approximately in same relative position as in most megatheriines, rather than elevated as in Megatherium species; posterolateral opening of the mandibular canal on anterior edge of base of ascending ramus, posterior to m4, and visible in lateral view; ascending ramus begins approximately at level of distal surface of m4, so that m4 was uncovered in lateral view; inclination of base of ramus suggests that anterior margin of coronoid process inclined less steeply than in other megatheriines; symphysis begins posteriorly well anterior to m1; humerus possesses very prominent deltopectoral crest; in the ulna, proximolateral articular facets face nearly laterally, and olecranon process compressed mediolaterally; in the femur, patellar trochlea contiguous distally with medial and lateral articular facets for the tibia; in the astragalus, navicular facet relatively dorsal, with plane of discoid facet intersecting its middle portion; and fibular facet elongated, reaching the ectal facet.