Argüello's first defense came against Venezuelan featherweight champion Leonel Hernández. Once again, Argüello fought in enemy territory, as the fight took place in Caracas. Nevertheless, Argüello made short work of his challenger, stopping him by technical knockout in the 8th round. His first defense in Nicaragua was against Rigoberto Riasco. Argüello dominated once again, this time stopping Riasco in the second round. Next up for Argüello would be Royal Kobayashi, a highly touted Japanese challenger who was undefeated until then. After a tense, close start Argüello's relentless body-punching broke Kobayashi halfway through the fifth round, with the challenger dropping to the canvas twice.