Thirteen years ago, on the first season of the CBS reality competition “Big Brother,” one houseguest was William Collins, also known as Will Mega, an intensely political black man who proved polarizing in the house for his sometimes confrontational talk about race. It was a novelty, this sort of conversation, in network prime time, and in the eyes of Americans, perhaps not a welcome one: the audience voted Mr. Collins off the show first.

In the years since, “Big Brother” casts have displayed token diversity, but rarely has race been a subject of conversation, partly because of the producers’ apparent belief that it will arise only when members of a minority group are present and vocal, a scenario they’ve largely avoided.

This season, though, race has become the dominant narrative thanks to a handful of white cast members who’ve turned the show, which runs three nights a week, into a rare opportunity to watch white privilege and unconscious racism in the field. It may be occurring in a sealed-off space, but it’s feasting on the oxygen of national network television.

Sunday’s episode was the most disturbing of the season so far. Aaryn Gries, a young white woman, flipped the mattress of Candice Stewart, one of the show’s black cast members, then taunted Ms. Stewart with an exaggerated, stereotypical voice: “What you gon’ do, girl?” and “Where’s yo class, girl?” She was soon backed up by another white cast member, GinaMarie Zimmerman, who screamed at Ms. Stewart, “You want the black to come out?”