During the last decade or so, Rosie O'Donnell - the former talk-show host and current suburban mom - has been many things to many musicals. She has been a showstopper in "Grease!," an attempted show-saver in "Seussical" and a show producer with "Taboo," the 2003 flop which left a $10 million dent in her bank account. She has been a host of the Tonys, roasted her critics, and generally made herself either beloved or belittled by everybody on Broadway.

But next month Ms. O'Donnell will step into a role that not only will give her supporters and detractors plenty to talk about, but also surely ranks as one of the boldest bits of replacement casting in Broadway history. Beginning on Sept. 20 at the Minskoff Theater, Ms. O'Donnell, a brassy Irish-American comedian from Long Island, will play Golde, the long-suffering wife and devoted mother in "Fiddler on the Roof," the 1964 classic of life in the shtetl.

That Ms. O'Donnell will play opposite Harvey Fierstein - whose casting as the daughter-laden dairyman Tevye last winter also raised a few eyebrows among musical-theater traditionalists - makes it truly an Only on Broadway moment, where two openly gay and outspoken actors will play husband and wife in a musical all about the breaking of traditions.

Ms. O'Donnell was not available for comment yesterday, but in a news release announcing her casting - complete with a photograph of the actress in a shawl and head scarf - Mr. Fierstein said he was thrilled.

"As a 20-year friend of Rosie's, I couldn't be happier that we will finally share a stage together," he said in the announcement. "With her understanding of Fiddler's important place in theatrical history, I am positive she will uphold that tradition excitingly."

The show's producers - who called the casting "a match made in musical-theater heaven" in the release - are probably no less excited; in her Broadway experience ("Grease!" in the mid-1990's, "Seussical" in 2001), Ms. O'Donnell proved to be a solid draw at the box office. The casting of Mr. Fierstein, who took over for Alfred Molina in January, also sparked several months of strong sales for the production.

But Ms. O'Donnell's casting, which had been rumored along Broadway for several days, comes at a particularly critical time for "Fiddler," which has been struggling to fill seats in recent weeks. During the last month, the show has been drawing only between 55 and 65 percent of its capacity at the Minskoff. On Aug. 1 the show's producers - including the Nederlander Organization, which owns the Minskoff - announced that it would close "Fiddler" on Jan. 8; it was unclear yesterday whether Ms. O'Donnell's casting would change that, but producers were certainly hoping for a prolonged boost in sales.

Since opening in February 2004, the $8 million production - the musical's fourth revival on Broadway - has recouped about half its capitalization. The original production - with music by Jerry Bock, lyrics by Sheldon Harnick and a book by Joseph Stein - ran for nearly eight years.

Reached at home yesterday, Mr. Harnick - who had not yet heard that Ms. O'Donnell was officially cast - said he felt she had all the right qualities to play Golde.

Ms. O'Donnell "is certainly strong, and down to earth, with a certain peasanty quality to her," he said, adding that Golde "has been played so many different ways."

He continued: "She loves deeply. She loves her children. She's a survivor. There's nothing essentially romantic or sentimental. She's a realist. And all of those qualities certainly fit Rosie, from what I know from her work."

The casting is not the first surprising development for this revival, being directed by David Leveaux. In addition to casting Mr. Fierstein, who received mixed reviews, the show also fended off early attacks from some critics and theater columnists that Mr. Leveaux, an Englishman, had drained the production of its Jewish character, a charge the director and the show's creators strongly denied.

Yesterday Mr. Harnick said that he hoped the production's most recent twist would pay off. "Harvey I had grave doubts about and he's marvelous," Mr. Harnick said. "So hopefully lightning will strike twice."