LOS ANGELES, Aug. 5 (Xinhua) -- The third installment of the Bourne movie franchise opened strongly at the box office in North American theaters this weekend, taking in an estimated 70.2 million dollars in ticket sales in the three-day period.

Universal's "The Bourne Ultimatum" is on the track to become the biggest film opening ever for the month of August, surpassing the 67.4 million-dollar opening posted by "Rush Hour 2" the same weekend in 2001, according to box office tracking firm Media by Numbers Sunday.

Meanwhile, the third movie starring Matt Damon as an amnesiac CIA assassin fighting to retrieve his lost past far exceeded the opening ticket sales generated by the first two films of the series.

"The Bourne Identity" debuted in North America with 27.1 million dollars in June 2002 and "The Bourne Supremacy" with 52.5 million dollars in July 2004. But the two films went on to gross a total of 485 millions worldwide.

Last weekend's box office leader, 20th Century Fox's "The Simpsons Movie" slipped to second with about 25.6 million dollars in sales in its second weekend, earning the studio some 128.5 million dollars in two weeks.

Another new release, Disney's canine comedy "Underdog," took third place with 12 million dollars in estimated sales.

Universal's comedy "I Now Pronounce You Chuck and Larry" and New Line's musical "Hairspray" ranked fourth and fifth at box office over the weekend, with 10.5 million and 9.3 million dollars in ticket sales respectively.

Bolstered by a series of blockbusters since early May, Hollywood has managed to maintain its box office health this summer. The overall ticket sales of all films in theaters this weekend are up about 25 percent over the same period last year.