3:45 PM PT -- A rep for Ultra tells TMZ, Deadmau5 will replacing Avicii on Saturday.
Avicii has officially canceled his huge performance at Ultra this weekend -- and it's all his gallbladder's fault ... TMZ has learned.
DJ people tell us Avicii is still in the Miami hospital where he was rushed Thursday night for severe abdominal pains, nausea, and fever. We're told he has a blocked gallbladder and needs it removed.
As a result, Avicii has been forced to cancel all his commitments this week -- including his highly anticipated Ultra performance.
Avicii's bummed he's missing his big show -- but obviously ... the guy has no choice.