Megacyllene (Megacyllene) cryptofrasciata n. sp. from Argentina is described and illustrated. M. quinquefasciata (Melzer, 1931), and Megacyllene rotundicollis Zajciw, 1963 are transferred from the subgenus Megacyllene Casey 1912 to Sierracyllene Tippmann, 1960. Megacyllene (Sierracyllene) tafivallensis n. sp. is described from northwestern Argentina. Dexithea spixii (Laporte & Gory, 1836), and Plagionotus latreillei (Laporte & Gory, 1836) are transferred to Megacyllene (sensu stricto), excluding Dexithea, and Plagionotus from the South American fauna of Clytini. Neoclytus famelicus (Burmeister, 1865) is synonymized with N. ypsilon Chevrolat, 1861. Additional new records of Clytini from Argentina, Paraguay, and Ecuador are also presented here. A key for subgenera and species of Megacyllene is included, with distribution maps for Argentina and nearby countries.
