NBC and Fox had their lowest ratings among young viewers since at least 1987, when Nielsen Media Research began precise people meter measurements. ABC tied its record-low rating among those same 18-to-49-year-old viewers, bottoming out at the same level it hit two other weeks this summer.

All three networks chase that youthful demographic almost exclusively.

The average of 4.2 million viewers of all ages who tuned in to NBC in prime time last week was the network's lowest in decades. The other networks' weekly averages turned out to be the third-lowest for ABC, fourth-lowest for Fox and ninth-lowest for CBS, according to Nielsen.

Judging by a schedule clogged with even more reruns than usual, it's obvious the broadcasters didn't put much emphasis on the week. They're more worried about the fall season, which begins the last week of September. But last week's troubles meant fewer viewers were available to watch network promos for their new series.

The only program to deliver more than 10 million viewers last week was a rerun of "Two and a Half Men" on CBS.

For the week, CBS dominated with 6.8 million viewers (4.5 rating, 8 share). Fox had 4.4 million viewers (2.8, 5), ABC had 4.3 million (2.9, 5), NBC had 4.2 million (2.8, 5), the CW had 1.8 million (1.2, 2), My Network TV had 920,000 (0.7, 1) and ION had 590,000 (0.4, 1).

Among the Spanish-language networks, Univision averaged 3.2 million viewers (1.7 rating, 3 share), Telemundo had 950,000 (0.5, 1), TeleFutura had 680,000 (0.4, 1) and Azteca had 190,000 (0.1, 0).

Brian Williams of NBC's "Nightly News" took advantage of Charles Gibson's vacation on ABC to climb atop the evening news ratings. NBC averaged 7.8 million viewers last week (5.3 rating, 11 share), ABC's World News" had 7.6 million viewers (5.1, 11) and the "CBS Evening News" had 5.9 million (4.1, 9).

A ratings point represents 1,130,000 households, or 1 percent of the nation's estimated 112.8 million TV homes. The share is the percentage of in-use televisions tuned to a given show.

For the week of Aug. 27-Sept 2, the top 10 shows, their networks and viewerships: "Two and a Half Men," CBS, 10.31 million; "The Power of Ten" (Tuesday), CBS, 9.89 million; "CSI: Miami," CBS, 9.09 million; "CSI: NY," CBS, 8.9 million; "Criminal Minds," 8.46 million; "The Power of Ten" (Wednesday), CBS, 8.34 million; "Big Brother 8" (Tuesday), CBS, 8.14 million; "House," Fox, 7.95 million; "CSI: Crime Scene Investigation," CBS, 7.77 million; "Rules of Engagement," CBS, 7.62 million.

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