For the first time, a major Hollywood film will hit the $20 threshold at the box office, as movie-theater owners test the public's ability to absorb ever higher ticket prices.

Several theaters will charge $20 per adult ticket to IMAX showings of the animated 3-D family film "Shrek Forever After," the fourth "Shrek" installment from DreamWorks Animation. The theaters include the AMC theater in Manhattan's Kips Bay neighborhood, AMC Loews 34, AMC Loews Lincoln Square and AMC Empire 42nd Street.

The increases weren't officially announced, but were reflected in prices posted Wednesday on movie-ticketing Web sites such as Fandango.com and tracked by BTIG LLC media analyst Richard Greenfield.

"With the state of the economy remaining questionable, we worry pricing is simply moving up too quickly," cautioned Mr. Greenfield in a research blog post, adding that he was especially concerned about how quickly children's ticket prices are increasing. "The danger is scaring consumers away from the movie theaters."

This weekend's price increase come less than eight weeks after theater operators instituted some of the steepest hikes in a decade. Those increases in late March—in some cases of as much as 26%—varied theater to theater and focused on 3-D and IMAX showings of another DreamWorks Animation title, "How to Train Your Dragon." The same AMC theater in Manhattan charged $19.50 for an IMAX showing of "Dragon."