Even the biggest show on TV, “American Idol,” isn’t immune from the ratings declines that have been plaguing broadcast’s biggest hits this year. With only two weeks left until the sixth-season finale, “Idol’s” Tuesday edition hit a season low last night.

“Idol” averaged an 8.9 adults 18-49 rating, according to Nielsen overnights, 11 percent below its previous Tuesday low of 10.0 two weeks ago. It was down 13 percent from a 10.2 last week.

Season to date, Tuesday “Idol” is averaging a 12.3. On the same night last year, “Idol” averaged an 11.4.

Of note, “Idol” ratings have been adjusting several tenths upwards when final ratings are released the past few weeks, and previous weeks’ ratings cited are finals, not overnights. But the declines are undeniable.

The show has been losing audience steadily for several weeks, though it still towers above the rest of the networks’ offerings, last night all but obliterating the competition in the 8 p.m. hour.

This may simply be part of the ratings decline that seems to have turned into an epidemic, what with big hits like “Desperate Housewives,” “Lost,” “CSI,” “ER,” “24” and more all hitting series or season lows over the past six weeks. Everything from digital video recorders to the early daylight savings has been blamed.

Or it may reflect viewers’ lukewarm feelings toward the “Idol” finalists or last night’s Bee Gees theme.

But there was some good news for Fox: Because of or in spite of last night’s low for “Idol,” “House” actually achieved its best-ever retention out of the singing show, 100 percent.

Thus Fox easily finished first last night among viewers 18-49 with an 8.9 average rating and a 25 share. ABC and CBS tied for second at 2.8/8, with NBC fourth at 2.4/7, Univision fifth at 1.7/5 and CW sixth at 1.4/4.
 
At 8 p.m. Fox led with an 8.9 rating for “American Idol,” followed by CBS with a 3.1 for “NCIS.” CW was third that hour with a 1.8 for “Gilmore Girls,” Univision fourth with a 1.7 for “La Fea Mas Bella,” ABC fifth with a very unpromising 1.6 average for the hour-long season finale of “George Lopez” and NBC sixth with a 1.4 for “Dateline.”
 
Fox led at 9 p.m. with another 8.9 rating for “House.” ABC jumped to second with a 4.1 for a “Dancing with the Stars” results show, with CBS third with a 3.1 for “The Unit,” and Univision and NBC tied for fourth at 2.0, Univision for “Destilando Amor” and NBC for “Law & Order: Criminal Intent.” CW was sixth that hour with a 0.9 for “Veronica Mars.”
 
NBC took the lead at 10 p.m. with a 3.7 for “Law & Order: Special Victims Unit,” while ABC finished second with a 2.7 for “Boston Legal,” CBS third with a 2.2 for a repeat of “Without a Trace” and Univision fourth with a 1.4 for “Nuestra Belleza Latina.”
 
Among households, Fox led the night with a 13.7 average rating and a 22 share. CBS was second at 7.1/12, ABC third at 7.0/11, NBC fourth at 5.3/9, Univision fifth at 2.2/4 and CW sixth at 1.9/3.