The first record of milk teeth of South American fossil procyonids comes from the Late Miocene-Early Pliocene at "Farola Monte Hermoso," Buenos Aires Province, Argentina. Five extant genera of Procyonidae inhabit South America (Bassaricyon Allen, Nasuella Hollister, Potos Geoffroy Saint-Hilaire and Cuvier, Procyon Storr, and Nasua Storr). Of these only Procyon and Nasua are present in the fossil record (Late Pleistocene-Holocene), in several localities in Brazil, Uruguay, and Bolivia. In addition, six other fossil genera were named, but only two are considered valid: Cyonasua and Chapadmalania. Thus, Cyonasua encompasses ten formally named species and Chapadmalania two. The new specimen, MLP 09-X-5-1, is assigned to cf. Cyonasua. In addition, anatomical evidence implies a much more carnivorous diet in Late Miocene-Early Pleistocene procyonids than that of extant South American taxa. Finally, I examine and discuss the "competitive displacement" hypothesis regarding the extinction of native marsupial carnivores after the arrival of immigrant placental carnivores in South America.