The victory over Rua was followed by Arona's first defeat in PRIDE, losing his fourth fight to Quinton Jackson in a spectacular knockout. Arona, outsized for the first time in the promotion, saw Jackson powering out of his holds, so he elected to stay in his guard to minimize damage. He seemed to knock out Jackson with an upkick, but as it wasn't so, he followed with a triangle choke, only for Jackson to suddenly lift him up and counter it with a powerbomb. The slam knocked Arona out and led the referee to stop the match, declaring Jackson the winner. After the bout, Arona protested and claimed that an illegal headbutt he received by Jackson at the end of the slam was what knocked him out; footage of the fight seems to support this, but it was never officially acknowledged or investigated.