Starring Ben Stiller, Jack Black and Robert Downey Jr., action comedy "Tropic Thunder" maintained its No.1 position at the box office for a third-straight weekend, grossing $11.5 million in ticket sales for Viacom Inc.’s Dreamworks SKG studio, while 20th Century Fox's Van Diesel sci-fi actioner "Babylon A.D." secured the second spot with $9.7 million, Variety reported.

Far from a blockbuster, the R-rated "Tropic Thunder," which cost more than $90 million to produce, has hung tough in the market, ultimately cashing more than $80 million in domestic grosses.

"For this movie to finish first three weeks in a row is really impressive,'' said Paul Dergarabedian, president of Media By Numbers, in a telephone interview. "The great word-of-mouth helped, and people are obviously in the mood for comedy.''

Directed by Ben Stiller, the film is far from being politically correct, thus it became the target of various complaints and calls for a boycott by several groups that were against the film’s portrayal of mentally disabled people. The movie mocks several other categories of people, from Vietnam War veterans and drug addicts to overweight people.

Although theater traffic is relatively slow during Labor Day weekend, this year the box office was particularly sluggish five new films competing for attention in domestic theaters. None of which even managed to reach the $10 million mark.

"It was an underwhelming end to a phenomenal summer," Dergarabedian added.

However, this summer's reigning blockbuster "The Dark Knight," raked in a total of $8.75 million pushing its whole domestic haul to more than $502 million, only the second film in history - after James Cameron's "Titanic" - to top the half-billion-dollar mark.

Overture's Don Cheadle-Guy Pearce political thriller "Traitor" opened solidly son the strength of adult moviegoers, grossing an estimated $7.9 million from 2,054 theaters through Sunday for a strong per location average $3,846 and a five-day cume of $9.5 million. Film came in No. 5 for the weekend.

Another new release, "Disaster Movie,'' from Lions Gate Entertainment Corp., opened seventh with $6.17 million while "Mamma Mia!'' from Universal, had earnings of $4.42 million and was eighth. Also among the five new films, MGM's comedy romp "College" only came in No. 15, grossing an estimated $2.1 million from 2,123 runs.