Channel 4's live Big Brother launch show attracted an audience of 5.1 million people on Wednesday, initial figures suggest.
The show, which clashed with BBC One's Junior Apprentice was the most watched programme at 2100 BST.
Last year's series opener was seen by 5.4 million people.
For its last ever run on Channel 4, Big Brother welcomed 14 housemates, including a Beyonce look-a-like and a Christian minister.
The contestants were shortlisted from 80 hopefuls, with the last entrant picked at random.
Mario, 28, from Essex had his name drawn from a tombola by presenter Davina McCall and was provided with a mole costume.
Big Brother set him the series' first task, telling him he must commit "a series of dastardly deeds" against the other housemates.
If they guess what he is up to, Mario will be evicted.