A NEW SPECIES OF RHEA (AVES: RHEIDAE) FROM THE LATE MIOCENE OF THE MESOPOTAMIA IN ARGENTINA. A new species of rhea, Pterocnemia mesopotamica sp. nov. (Aves: Rheidae), from the late Miocene of Mesopotamia in Argentina is described. Specimens come from the base of the Ituzaingó Formation (= “Conglomerado osífero” or “Mesopotamiense”), which crops out along the cliffs of the Paraná river near the homonimous city in Entre Ríos Province. Pterocnemia mesopotamica sp. nov. is a slender and small-bodied bird, similar in size to the lesser rhea or choique (P. pennata), and characterized by the marked divergence of the tarsometatarsal trochleae. Fragmentary remains of a femur and a humerus, identified as Rheidae indet., are also reported from the same locality and horizon. An isolated tarsometatarsus, coming from the Aisol Formation in Mendoza Province, is referred to Pterocnemia cf. P. mesopotamica sp. nov. If this record is confirmed, the biochron of the new taxon would extend back from the late Miocene (Huayquerian Age) to the middle Miocene (Friasian Age).