On July 22, 2006, Castillo lost his title to Nobuo Nashiro in Japan. The fight was stopped in the 10th round as Castillo was bleeding heavily from cuts. As the cuts were caused by punches, Nashiro won the fight by technical knock-out. Nashiro, a young boxer with only 7 professional fights, surprised Castillo with an effective aggressive style in the early rounds. Castillo was winning later rounds - had the fight gone to the scorecards after the 9th round, it would have been a draw, as one judge had Nashiro winning, one had Castillo winning, and one had it even.