Tibau made his UFC debut at welterweight against Nick Diaz . Tibau started strong and was taking Diaz down efficiently while avoiding submissions using Brazilian Jiu Jitsu . In the second round he got tired and lost by TKO due to strikes. Tibau took on Jason Dent at UFC 68 . Tibau won by unanimous decision. He then took a one fight hiatus from the company and returned to defeat Jeff Cox by submission due to an arm-triangle choke. Tibau then fought undefeated Brit Terry Etim , outwrestling his opponent for a unanimous decision.
In July 2007, Sherk successfully defended his title against Hermes França at UFC 73 .  After the fight, the California State Athletic Commission (CSAC) announced that Sherk had tested positive for Nandrolone , a banned steroid , in a urine test the day prior to the fight. After testing positive for 12 ng/ml of Nandrolone, Sherk was fined $ 2,500 and suspended from competing in California for one year.  Sherk's suspension was reduced to six months after appealing his allegations on December 4, 2007, making him eligible to fight in early January 2008.  Sherk argued that errors were made in lab testing procedures. He asserted that the lab had failed to properly test the vials used in earlier, positive tests for any remaining steroid content, which may have resulted in his sample becoming contaminated.  Quest Diagnostics and the CSAC denied any errors were made.  Following the CSAC's decision to uphold the results of the positive steroid test, the UFC officially stripped Sherk of his Lightweight Championship.  At UFC 80 , . Penn defeated Joe Stevenson to win the vacant title.  After the fight, UFC president Dana White announced that Penn's first title defense would be against Sherk, who was reinstated by UFC in April 2008, and has tested clean ever since.  Sherk and Penn fought at UFC 84 on May 24, 2008. Sherk lost by TKO in the third round after failing to answer the fourth round bell.