New information on the vocalizations and behavior of populations of the Olive-backed Foliage-gleaner (Automolus infuscatus), along with analysis of biometric and plumage characters, reveal that it actually consists of at least two biological species: a form that is restricted to southeastern Amazonia, south of the Amazon and east of the Rio Madeira; and a group that occupies much of the remainder of humid lowland forest in Amazonia and the Guianan region. The southeastern form, although cryptically similar morphologically to other southern Amazonian subspecies of Automolus infuscatus, is highly differentiated in all vocal characters. Its distribution overlaps the Rondônia, Pará, and Belém centers of endemism. The remainder of the populations currently considered part of the infuscatus group are further differentiated into two subgroups: a northern Guianan group and a southern-western group, the member taxa of which are weakly differentiated from one another, but which differ from members of the other group in plumage, biometric, and vocal characters. The significance of these differences with respect to species limits may depend on the species concept used. Current evidence would support recognition of the two groups as separate phylogenetic species in addition to the southeastern species, but is considered insufficient to support recognition of more than two species under a biological species concept.