Reuters reports that in the biggest ratings matchup yet between marquee "reality" shows this summer, the CBS premiere of "Big Brother 3" went up against "American Idol" on Fox and came out on top -- but not by much.

According to figures on Thursday from Nielsen Media Research, the latest group of CBS "house guests" were No. 1 in household ratings and total audience in their 9 p.m. Wednesday debut opposite Fox's teen idol wannabes. However, the margin of victory was fairly slim by both measures -- 6.0 to 5.3 in the ratings, and 9.2 million vs. 9 million in average number of viewers.

In the key demographic of adult viewers under 50, the group most prized by advertisers, CBS and Fox were tied with a 4.1 rating. Furthermore, "American Idol" bested "Big Brother" in two other young demographics -- teen viewers and adults 18-34 -- that make up Fox network's target audience for its show.

The shows will not normally air head-to-head. This week's direct match-up came because Fox's normal hour-long installment of "Idol" on Tuesday was preempted by baseball. Rival reality show, NBC's stunt-oriented "Dog Eat Dog," ranks No. 3 for the summer in the adults-under-50 crowd.