Summer fizzled out as the Labor Day weekend saw Warner Bros' The Dark Knight finally cross $500m at the domestic box office in its seventh weekend in release.

DreamWorks-Paramount's comedy Tropic Thunder stayed atop the charts despite a concerted challenge from the sci-fi adventure Babylon AD.

Tropic Thunder added an estimated $11.5m to raise its running total to $83.8m after three weekends, while Babylon AD, which is released in North America through Fox and stars Vin Diesel as a mercenary charged with escorting precious cargo from Russia to the US, debuted in second place on $9.7m.

The Dark Knight climbed one place to third on $8.8m and easily ranks as the second biggest North American earner in history on $502.4m. Figures are not adjusted for inflation.

The terrorism thriller Traitor starring Don Cheadle as an ambiguous operative and Guy Pearce as the FBI agent on his trial launched in fifth place on $7.9m through Overture Films. Paramount Vantage sold overseas rights. The comedy spoof Disaster Movie launched through Lionsgate in seventh place on $6.2m.

Woody Allen's Vicky Cristina Barcelona climbed one place to 10th through The Weinstein Company on $3m for $12.8m after three weekends.

Summer box office figures will emerge later this week and the season could come close to last year's $4.18bn record.

Next weekend's wide release is the action thriller Bangkok Dangerous through Lionsgate starring Nicolas Cage.

Estimated Top 10 North America Aug 29-31, 2008
Film (Dist)/Int'l dist/Est wkd gross/Est total to date

1 (1) Tropic Thunder (DreamWorks-Paramount) PPI $11.5m $83.3m
2 (-) Babylon AD (Fox) StudioCanal $9.7m -
3 (4) The Dark Knight (Warner Bros) WBPI $8.8m $502.4m
4 (2) The House Bunny (Sony) SPRI-various $8.3m $27.9m
5 (-) Traitor (Overture) Paramount Vantage Int'l $7.9m $9.4m
6 (3) Death Race (Universal) UPI $6.2m $23.1m
7 (-) Disaster Movie (Lionsgate) Summit Int'l $6.2m -
8 (9) Mamma Mia! (Universal) UPI $4.4m $131.5m
9 (6) Pineapple Express (Sony) SPRI $3.4m $79.8m
10 (11) Vicky Cristina Barcelona (TWC) Wild Bunch $3m $12.8m