Micromorphological differences between hairs of the two-toed sloth (Choloepus hoffmanni) and of the three-toed sloth (Bradypus variegatus) were observed by means of scanning electron microscopy. Hairs of the two-toed sloth had longitudinal grooves which exposed a uniform cortex when viewed in cross section. Hairs of the three-toed sloth have irregular transverse fissures and a cortex that contains scattered fusi. These morphological differences support the placement of the two sloth genera into separate familes. Hairs of four other xenarthran species are al so illustrated.Los pelos del perezoso de dos dedos, Choloepus hoffmanni, tienen estrías longitudinales y una corteza uniforme (vista en corte transversal). Los pelos del perezoso de tres dedos, Bradypus variegatus, tienen fisuras transversales irregulares y una corteza con "fusi" diseminados. Las diferencias macromorfológicas apoyan la separación de ambos géneros en familias diferentes.