Three new genera and three new species of Baetidae (Insecta: Ephemeroptera) are described from the Andes and Patagonia. Larvae of Andesiops, new genus, are distinguished by the apically bifid right prostheca and two rows of denticles on the tarsal claws. Larvae of Deceptiviosa, new genus, are distinguished by the apically bifid right prostheca and small subapical denticle on the tarsal claws. Larvae of Nanomis, new genus, are distinguished by the apically acute maxillary palp segment 2, smooth posterior margin of the terga, and paddle-shaped subapical seta on the hindtibiae. Adults of Andesiops and Deceptiviosa are similar, but distinguished from other South American baetids by having hindwings with a small costal process in the basal one third and three lon gitudinal veins with the middle vein being forked, and an elongate and ellipsoidal genital forceps segment 3. Larvae and adults of Andesiops peruvianus, new combination, are redescribed, and additional locale data are provided for Argentina, Bolivia, Ecuador, and Venezuela. Deceptiviosa ardua, new species, is described from larvae from Chile, and D. torrens, new species, is described from larvae and adults from Chile and Argentina. Nanomis galera, new species, is described from larvae from Argentina, Colombia, Ecuador, and Peru.