"Cowboys & Aliens" and "The Smurfs" each brought in an estimated $36.2 million in gross U.S. receipts to tie for No. 1 during the weekend.

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Box Office Mojo said studios may have to wait until Monday, when the actual receipts are counted, to determine who won.

"Cowboys," with Daniel Craig and Harrison Ford, was shown in 3,750 theaters; "Smurfs," with Neil Patrick Harris and the animated little blue creatures, was shown in 3,395 theaters.

All studio estimates of gross box office receipts are via Box Office Mojo.

"Captain America: The First Avenger" was a healthy third with more than $24.9 million; "Harry Potter and the Deathly Hallows Part 2" a robust fourth with more than $21.92 million; "Crazy, Stupid, Love" fifth with $19.3 million; "Friends with Benefits" sixth with $9.3 million; "Horrible Bosses" seventh with $7.1 million; "Transformers: Dark of the Moon" eighth with $5.97 million; "Zookeeper" ninth with $4.2 million.

"Cars 2" was 10th with more than $2.3 million.